Long-Term Debt (Narrative) (Details) - USD ($) | Mar. 15, 2019 | Jul. 03, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018 | Jul. 31, 2019 | Mar. 31, 2019 | Mar. 19, 2018 |
Debt Instrument [Line Items] | | | | | | | | |
Payments to retire debt instruments | | | $ 2,479,000,000 | | | | | |
Line of credit facility, current borrowings | | | 749,000,000 | | | | | |
Loss on extinguishment of debt | | | (20,000,000) | $ 31,000,000 | | | | |
Proceeds from long-term debt borrowings | | | 1,650,000,000 | $ 1,840,000,000 | | | | |
Remaining outstanding principal | | | 17,520,000,000 | | $ 17,400,000,000 | | | |
Letter of Credit [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Line of credit facility, current borrowings | | | 101,000,000 | | | | | |
Senior Unsecured Debt [Member] | Senior Note Due 3/15/2019 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Payments to retire debt instruments | | | $ 348,000,000 | | | | | |
Debt instrument, maturity date | | | Mar. 15, 2019 | | | | | |
Interest Rate | | | 7.125% | | | | | |
Senior Unsecured Debt [Member] | Senior Notes Due 2020 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Remaining outstanding principal | | | $ 227,000,000 | | | | | |
Senior Unsecured Debt [Member] | Senior Notes Due 2021 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Remaining outstanding principal | | | 309,000,000 | | | | | |
Secured Debt [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Remaining outstanding principal | | | $ 5,715,000,000 | | $ 5,246,000,000 | | | |
Secured Debt [Member] | First Lien Notes Due 4/1/2027 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, face amount | | | | | | | $ 1,650,000,000 | |
Debt instrument, maturity date | | | Apr. 1, 2027 | | | | | |
Interest Rate | | | 8.00% | | | | 8.00% | |
Remaining outstanding principal | | | $ 1,650,000,000 | | | | | |
Secured Debt [Member] | Second Lien Notes Due 4/1/2026 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, face amount | | | | | | | | $ 1,600,000,000 |
Debt instrument, maturity date | | | Apr. 1, 2026 | | | | | |
Interest Rate | | | 8.50% | | 8.50% | | | 8.50% |
Remaining outstanding principal | | | $ 1,600,000,000 | | $ 1,600,000,000 | | | |
Secured Debt [Member] | Term Loan Due 3/31/2021 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Interest Rate | | | | | 5.28% | | | |
Remaining outstanding principal | | | | | $ 1,402,000,000 | | | |
Secured Debt [Member] | Term Loan Due 10/12/2021 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Oct. 12, 2021 | | | | | |
Interest Rate | | | | | 7.405% | | | |
Remaining outstanding principal | | | | | $ 239,000,000 | | | |
Secured Debt [Member] | Term Loan Due 6/15/2024 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Jun. 15, 2024 | | | | | |
Interest Rate | | | 5.80% | | 6.28% | | | |
Remaining outstanding principal | | | $ 1,703,000,000 | | $ 1,716,000,000 | | | |
Unsecured Debt [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Remaining outstanding principal | | | $ 10,949,000,000 | | $ 11,297,000,000 | | | |
Unsecured Debt [Member] | Senior Note Due 7/1/2021 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Jul. 1, 2021 | | | | | |
Interest Rate | | | 9.25% | | 9.25% | | | |
Remaining outstanding principal | | | $ 89,000,000 | | $ 89,000,000 | | | |
Unsecured Debt [Member] | Senior Note Due 3/15/2019 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Mar. 15, 2019 | | | | | |
Interest Rate | | | | | 7.125% | | | |
Remaining outstanding principal | | | | | $ 348,000,000 | | | |
Unsecured Debt [Member] | Senior Note Due 4/15/2020 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Apr. 15, 2020 | | | | | |
Interest Rate | | | 8.50% | | 8.50% | | | |
Remaining outstanding principal | | | $ 172,000,000 | | $ 172,000,000 | | | |
Unsecured Debt [Member] | Senior Note Due 9/15/2020 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Sep. 15, 2020 | | | | | |
Interest Rate | | | 8.875% | | 8.875% | | | |
Remaining outstanding principal | | | $ 55,000,000 | | $ 55,000,000 | | | |
Unsecured Debt [Member] | Senior Note Due 9/15/2021 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Sep. 15, 2021 | | | | | |
Interest Rate | | | 6.25% | | 6.25% | | | |
Remaining outstanding principal | | | $ 220,000,000 | | $ 220,000,000 | | | |
Secured Subsidiary Debt [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Remaining outstanding principal | | | 106,000,000 | | 107,000,000 | | | |
Unsecured Subsidiary Debt [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Remaining outstanding principal | | | $ 750,000,000 | | $ 750,000,000 | | | |
JP Morgan Combined Credit Facility [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt Instrument, Acceleration Clause | | | The maturities of the Term Loan B and the Revolver, in each case if still outstanding, will be accelerated in the following circumstances: (i) if, 91 days before the maturity date of any series of Senior Notes maturing in 2020, 2023 and 2024, more than $500 million in principal amount remains outstanding on such series; or (ii) if, 91 days before the maturity date of the first series of Senior Notes maturing in 2021 or 2022, more than $500 million in principal amount remains outstanding, in the aggregate, on the two series of Senior Notes maturing in such year. | | | | | |
Days Prior To Maturity To Meet Threshold To Not Accelerate Debt | | | 91 days | | | | | |
Maximum Debt Threshold To Not Accelerate Debt | | | $ 500,000,000 | | | | | |
JP Morgan Revolving Credit Facility 2015 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Feb. 27, 2024 | | Feb. 27, 2022 | | | |
Line of credit facility maximum borrowing capacity | | | $ 850,000,000 | | | | | |
JP Morgan Revolving Credit Facility 2015 [Member] | London Interbank Offered Rate (LIBOR) [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Effective basis spread | | | 3.00% | | | | | |
JP Morgan Revolving Credit Facility 2015 [Member] | Secured Revolving Debt Due 2/27/2024 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Feb. 27, 2024 | | | | | |
Increased interest rate | 0.25% | | | | | | | |
JP Morgan Term Loan A [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, face amount | | | $ 1,740,000,000 | | $ 1,625,000,000 | | | |
Debt instrument, maturity date | | | Jun. 15, 2024 | | | | | |
JP Morgan Term Loan A [Member] | Senior Note Due 3/31/2021 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Mar. 1, 2021 | | | | | |
JP Morgan Term Loan B [Member] | Base Rate [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Interest rate margin | | | 2.75% | | | | | |
JP Morgan Term Loan B [Member] | London Interbank Offered Rate (LIBOR) [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Interest rate margin | | | 3.75% | | | | | |
JP Morgan Term Loan B [Member] | Term Loan Due 6/15/2024 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, maturity date | | | Jun. 15, 2024 | | | | | |
Proceeds from long-term debt borrowings | | $ 240,000,000 | | | | | | |
CoBank Term Loan 2016 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Debt instrument, face amount | | | | | $ 315,000,000 | | | |
Debt instrument, maturity date | | | Oct. 1, 2021 | | | | | |
Letter Of Credit, July 2019 [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Line of credit facility, current borrowings | | | | | | $ 20,000,000 | | |
Minimum [Member] | JP Morgan Term Loan A and Revolver [Member] | Base Rate [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Interest rate margin | | | 1.00% | | | | | |
Minimum [Member] | JP Morgan Term Loan A and Revolver [Member] | London Interbank Offered Rate (LIBOR) [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Interest rate margin | | | 2.00% | | | | | |
Maximum [Member] | JP Morgan Term Loan A and Revolver [Member] | Base Rate [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Interest rate margin | | | 2.00% | | | | | |
Maximum [Member] | JP Morgan Term Loan A and Revolver [Member] | London Interbank Offered Rate (LIBOR) [Member] | | | | | | | | |
Debt Instrument [Line Items] | | | | | | | | |
Interest rate margin | | | 3.00% | | | | | |