Quarterly report pursuant to Section 13 or 15(d)

Debt and Secured Revolving Line of Credit (Narrative) (Details)

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Debt and Secured Revolving Line of Credit (Narrative) (Details)
1 Months Ended 3 Months Ended 6 Months Ended
Aug. 02, 2018
USD ($)
Apr. 14, 2015
USD ($)
Sep. 30, 2016
USD ($)
day
Jun. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Jul. 01, 2017
USD ($)
Jun. 29, 2018
$ / shares
Dec. 30, 2017
USD ($)
Jun. 16, 2014
USD ($)
Feb. 26, 2014
USD ($)
Aug. 15, 2012
USD ($)
Debt Instrument [Line Items]                      
Aggregate principal amount repurchased [1],[2]         $ 15,000,000 $ 42,000,000          
Secured Revolving Line of Credit                      
Debt Instrument [Line Items]                      
Secured revolving line of credit, outstanding balance       $ 70,000,000 $ 70,000,000     $ 70,000,000      
Secured revolving line of credit, interest rate       5.50% 5.50%     4.75%      
Letters of credit outstanding, amount       $ 31,000,000 $ 31,000,000            
Secured revolving line of credit, remaining borrowing capacity       339,000,000 $ 339,000,000            
Repayment period         90 days            
Secured Revolving Line of Credit | Amended and restated loan and security agreement                      
Debt Instrument [Line Items]                      
Maximum borrowing capacity   $ 500,000,000                  
Maximum letters of credit available for issuance   45,000,000                  
Aggregate amount by which size of commitments may be increased (up to)   $ 200,000,000                  
Secured Revolving Line of Credit | Amended and restated loan and security agreement | Federal Funds Rate                      
Debt Instrument [Line Items]                      
Basis spread on variable rate   0.50%                  
Secured Revolving Line of Credit | Amended and restated loan and security agreement | LIBOR                      
Debt Instrument [Line Items]                      
Basis spread on variable rate   1.00%                  
2.125% Convertible Senior Notes due 2026                      
Debt Instrument [Line Items]                      
Stated interest rate     2.125%                
Principal amount     $ 805,000,000 805,000,000 $ 805,000,000     $ 805,000,000      
Outstanding aggregate principal amount     $ 805,000,000                
Remaining life         99 months            
Share price (in usd per share) | $ / shares             $ 14.99        
If-converted value in excess of principal       $ 703,000,000              
Effective interest rate       8.00% 8.00%            
2.125% Convertible Senior Notes due 2026 | Debt Instrument, circumstance 1                      
Debt Instrument [Line Items]                      
Threshold trading days | day     20                
Threshold consecutive trading days | day     30                
Threshold percentage of conversion price     130.00%                
2.125% Convertible Senior Notes due 2026 | Debt Instrument, circumstance 2                      
Debt Instrument [Line Items]                      
Threshold consecutive trading days | day     10                
Threshold percentage of conversion price     98.00%                
Conversion period after threshold period days | day     5                
6.75% Senior Notes due 2019                      
Debt Instrument [Line Items]                      
Stated interest rate                   6.75%  
Principal amount                   $ 600,000,000  
Outstanding aggregate principal amount       $ 153,000,000 $ 153,000,000            
Aggregate principal amount repurchased         14,000,000            
6.75% Senior Notes due 2019 | Subsequent Event                      
Debt Instrument [Line Items]                      
Aggregate principal amount settled with treasury stock $ 11,000,000                    
7.50% Senior Notes due 2022                      
Debt Instrument [Line Items]                      
Stated interest rate                     7.50%
Principal amount                     $ 500,000,000
Outstanding aggregate principal amount       347,000,000 347,000,000            
7.00% Senior Notes due 2024                      
Debt Instrument [Line Items]                      
Stated interest rate                 7.00%    
Principal amount                 $ 500,000,000    
Outstanding aggregate principal amount       310,000,000 $ 310,000,000            
Aggregate principal amount repurchased       $ 1,000,000              
[1] Amounts reflected adoption of ASU 2016-18, Statement of Cash Flows, Restricted Cash (Topic 230) beginning in the first quarter of 2018.
[2] Prior year amounts adjusted to reflect the retrospective application of ASU 2014-09, Revenue from Contracts with Customers. Refer to Note 1.