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Fair Value of Financial Instruments (Schedule of Estimated Fair Value of Financial Instruments, by Balance Sheet Grouping) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Assets:    
Available-for-sale securities $ 252 $ 299
Trading securities 37 80
Liabilities:    
Contingent Value Right, Fair Value Disclosure 2 1
Carrying Amount Measurement [Member]    
Assets:    
Cash and cash equivalents 563 238
Available-for-sale securities 252 299
Trading securities 37 80
Liabilities:    
Contingent Value Right, Fair Value Disclosure 2 1
Carrying Amount Measurement [Member] | Credit Facility, Type of Debt [Member] | Credit Facility, Name [Member]    
Liabilities:    
Credit Facility, Fair Value Disclosure 2,902 6,456
Carrying Amount Measurement [Member] | Senior Notes [Member] | Senior Notes at 8.0, Due 2019 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 1,922 1,920
Carrying Amount Measurement [Member] | Senior Notes [Member] | Senior Notes at 7.125, Due 2020 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 1,192 1,189
Carrying Amount Measurement [Member] | Senior Notes [Member] | Senior Notes at 6.875, Due 2022 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 2,943 2,932
Carrying Amount Measurement [Member] | Senior Secured Notes [Member] | Senior Secured Notes At 5.125 Due 2018 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure   698
Carrying Amount Measurement [Member] | Senior Secured Notes [Member] | Senior Secured Notes at 5.125, Due 2021 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 978 972
Carrying Amount Measurement [Member] | Senior Secured Notes [Member] | Senior Secured Notes at 6.25, Due 2023 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 3,061  
Carrying Amount Measurement [Member] | Receivables Facility and Other Debt, Type [Member] | Receivables Facility and Unsecured Debt [Member]    
Liabilities:    
Other Liabilities, Fair Value Disclosure 611 749
Estimate of Fair Value Measurement [Member]    
Assets:    
Cash and cash equivalents 563 238
Available-for-sale securities 252 299
Trading securities 37 80
Liabilities:    
Contingent Value Right, Fair Value Disclosure 2 1
Estimate of Fair Value Measurement [Member] | Credit Facility, Type of Debt [Member] | Credit Facility, Name [Member]    
Liabilities:    
Credit Facility, Fair Value Disclosure 2,826 6,370
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | Senior Notes at 8.0, Due 2019 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 1,637 1,615
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | Senior Notes at 7.125, Due 2020 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 897 917
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | Senior Notes at 6.875, Due 2022 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 1,729 2,102
Estimate of Fair Value Measurement [Member] | Senior Secured Notes [Member] | Senior Secured Notes At 5.125 Due 2018 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure   690
Estimate of Fair Value Measurement [Member] | Senior Secured Notes [Member] | Senior Secured Notes at 5.125, Due 2021 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 902 930
Estimate of Fair Value Measurement [Member] | Senior Secured Notes [Member] | Senior Secured Notes at 6.25, Due 2023 [Member]    
Liabilities:    
Notes Payable, Fair Value Disclosure 2,800  
Estimate of Fair Value Measurement [Member] | Receivables Facility and Other Debt, Type [Member] | Receivables Facility and Unsecured Debt [Member]    
Liabilities:    
Other Liabilities, Fair Value Disclosure $ 611 $ 749