Short-term Borrowings and Other Debt Obligations (Narrative) (Details)				3 Months Ended			9 Months Ended
	Jul. 03, 2019 USD ($)	Jun. 15, 2019 USD ($)	Feb. 28, 2018 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2019	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018
Short-term Debt [Line Items]
Debt instrument, covenant, senior leverage ratio, period two					2.5
Debt instrument, covenant, specified leverage and letter of credit ratio, period two							3.5
Debt instrument, covenant, asset coverage ratio							4.0
Debt Instrument, Covenant, Initial Current Asset Coverage Ratio							2.1
Bond Tender and Call Premium				$ 0		$ 0	$ 0	$ (34,000,000)
Repayment of Long-term Debt, Long-term Lease Obligation, and Capital Security							317,000,000	$ 471,000,000
364 Day Secured RCF [Member]
Short-term Debt [Line Items]
Short-term debt				0			0		$ 317,000,000
Committed letters of credit
Short-term Debt [Line Items]
Letters of Credit Outstanding, Amount				275,000,000			275,000,000
Committed Letters of Credit [Member]
Short-term Debt [Line Items]
Letters of Credit Outstanding, Amount				104,000,000			104,000,000
Revolving Credit Facility
Short-term Debt [Line Items]
Credit agreement, maximum capacity				305,000,000			305,000,000
Senior Notes, 9.875 Percent due 2025
Short-term Debt [Line Items]
Long-term Debt			$ 600,000,000
Senior Notes, 9.625% due 2019
Short-term Debt [Line Items]
Long-term Debt			425,000,000
Interest Payable			20,000,000
Repayment of Long-term Debt, Long-term Lease Obligation, and Capital Security			$ 475,000,000
DIP Credit Agreement
Short-term Debt [Line Items]
Proceeds from borrowings	$ 1,400,000,000
Repayments of long-term debt	616,000,000
Outstanding letters of credit	$ 271,000,000			$ 371,000,000			$ 371,000,000
Senior Notes
Short-term Debt [Line Items]
Interest payment on debt		$ 69,000,000
Senior Notes | Senior Notes, 7.75 Percent due 2021
Short-term Debt [Line Items]
Stated interest rate on debt		7.75%		7.75%			7.75%
Senior Notes | Senior Notes, 8.25 Percent due 2023
Short-term Debt [Line Items]
Stated interest rate on debt		8.25%		8.25%			8.25%
Senior Notes | Senior Notes, 6.80% due 2037
Short-term Debt [Line Items]
Stated interest rate on debt		6.80%		6.80%			6.80%
Senior Notes | Senior Notes, 9.875 Percent due 2025
Short-term Debt [Line Items]
Stated interest rate on debt			9.875%
Senior Notes | Senior Notes, 9.625% due 2019
Short-term Debt [Line Items]
Stated interest rate on debt			9.625%
Senior Notes | Senior Notes, 6.00% due 2018
Short-term Debt [Line Items]
Stated interest rate on debt										6.00%
LIBOR | Term Loan Agreement
Short-term Debt [Line Items]
Basis spread on variable rate (as a percent)							2.30%
LIBOR | 364-day credit agreement
Short-term Debt [Line Items]
Basis spread on variable rate (as a percent)							3.05%
LIBOR | Minimum | DIP Credit Agreement
Short-term Debt [Line Items]
Basis spread on variable rate (as a percent)	0.00%
LIBOR | Extending Lenders | Minimum | A&R Credit Agreement
Short-term Debt [Line Items]
Basis spread on variable rate (as a percent)							3.55%
LIBOR | Nonextending Lenders | Minimum | A&R Credit Agreement
Short-term Debt [Line Items]
Basis spread on variable rate (as a percent)							2.80%
Term Loan Agreement
Short-term Debt [Line Items]
Quarterly payment on debt				$ 12,500,000