Long-Term Debt (Details) (USD $)
3 Months Ended 1 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Apr. 30, 2015
Dec. 31, 2014
Debt and Available Credit Facility        
Long-term debt $ 4,726,000,000     $ 4,598,000,000
Deferred financing costs 85,000,000     90,000,000
Amortization of deferred financing costs 5,000,000 5,000,000    
Loss on extinguishment of debt 0 17,000,000    
$2.75 billion RBL credit facility - due May 24, 2019        
Debt and Available Credit Facility        
Face amount of debt instrument 2,750,000,000     2,750,000,000
Long-term debt 980,000,000     852,000,000
Borrowing capacity 2,750,000,000.00      
Aggregate amount of borrowings 980,000,000      
Letters of credit outstanding 82,000,000      
Remaining capacity 1,700,000,000      
$2.75 billion RBL credit facility - due May 24, 2019 | Subsequent event        
Debt and Available Credit Facility        
Borrowing capacity     2,750,000,000.00  
Term loans and notes are retired or refinanced prior to maturity to complete semi-annual redetermination     6 months  
$750 million senior secured term loan - due May 24, 2018        
Debt and Available Credit Facility        
Face amount of debt instrument 750,000,000     750,000,000
Long-term debt 496,000,000     496,000,000
Debt instrument issuance as a percentage of the par value 99.00%      
Effective interest rate (as a percent) 3.50%     3.50%
$750 million senior secured term loan - due May 24, 2018 | London Interbank Offered Rate LIBOR        
Debt and Available Credit Facility        
Reference rate for variable interest rate LIBOR      
Specified margin on reference rate (as a percent) 2.75%      
$750 million senior secured term loan - due May 24, 2018 | London Interbank Offered Rate LIBOR | Minimum        
Debt and Available Credit Facility        
Reference rate for variable interest rate LIBOR      
Reference rate floor for variable interest rate (as a percent) 0.75%      
$400 million senior secured term loan - due April 30, 2019        
Debt and Available Credit Facility        
Face amount of debt instrument 400,000,000     400,000,000
Long-term debt 150,000,000     150,000,000
Effective interest rate (as a percent) 4.50%     4.50%
$400 million senior secured term loan - due April 30, 2019 | London Interbank Offered Rate LIBOR        
Debt and Available Credit Facility        
Reference rate for variable interest rate LIBOR      
Specified margin on reference rate (as a percent) 3.50%      
$400 million senior secured term loan - due April 30, 2019 | London Interbank Offered Rate LIBOR | Minimum        
Debt and Available Credit Facility        
Reference rate for variable interest rate LIBOR      
Reference rate floor for variable interest rate (as a percent) 1.00%      
$750 million senior secured notes - due May 1, 2019        
Debt and Available Credit Facility        
Face amount of debt instrument 750,000,000     750,000,000
Interest rate (as a percent) 6.875%      
Long-term debt 750,000,000     750,000,000
$2.0 billion senior unsecured notes - due May 1, 2020        
Debt and Available Credit Facility        
Face amount of debt instrument 2,000,000,000     2,000,000,000
Interest rate (as a percent) 9.375%      
Long-term debt 2,000,000,000     2,000,000,000
$350 million senior unsecured notes - due September 1, 2022        
Debt and Available Credit Facility        
Face amount of debt instrument 350,000,000     350,000,000
Interest rate (as a percent) 7.75%      
Long-term debt $ 350,000,000     $ 350,000,000