Basis of Presentation and Significant Accounting Policies, Voluntary Reorganization under Chapter 11 Cases - Voluntary Reorganization under Chapter 11 Proceedings Narrative (Details) - USD ($) $ in Millions			12 Months Ended
	Oct. 18, 2019	Sep. 14, 2019	Dec. 31, 2019	Dec. 31, 2018	Nov. 25, 2019	Sep. 03, 2019	Aug. 15, 2019	Jan. 31, 2018
Restructuring Cost and Reserve [Line Items]
Equity Rights Offering amount backstopped	$ 463
Debtor-in possession financing, loans outstanding			$ 148	$ 0
Maximum borrowing capacity	629
Reorganization items			65	0
Liabilities subject to compromise			2,932	0
Liabilities subject to compromise, debt			2,780	0
Contractual interest expense on prepetition liabilities			235
Interest expense on prepetition liabilities recognized in Statement of Operations			211
Senior Unsecured Notes due September 1, 2022
Restructuring Cost and Reserve [Line Items]
Cash interest payment due, not paid						$ 7
Senior Secured Note due May 1, 2024
Restructuring Cost and Reserve [Line Items]
Principal amount issued								$ 1,100
Senior Secured Notes due February 15, 2025
Restructuring Cost and Reserve [Line Items]
Cash interest payment due, not paid							$ 40
Purchase price of new common shares	325
Principal amount issued	$ 138
Debtor-In-Possession Senior Secured Superpriority Revolving Credit Facility
Restructuring Cost and Reserve [Line Items]
Principal amount issued					$ 629
Debtor-in-possession financing, amount arranged					315
Debtor-in possession financing, remaining borrowing capacity			150
Debtor-in possession financing, loans outstanding			148	$ 0
Senior Unsecured Notes
Restructuring Cost and Reserve [Line Items]
Liabilities subject to compromise, debt			688
Letter of Credit
Restructuring Cost and Reserve [Line Items]
Debtor-in-possession financing, amount arranged					$ 50
Debtor-in-possession letter of credit outstanding			$ 17
Beneficial Owner | Senior Secured Notes due November 29, 2024
Restructuring Cost and Reserve [Line Items]
Percent of aggregate principal amount	52.00%
Beneficial Owner | Senior Secured Note due May 1, 2024
Restructuring Cost and Reserve [Line Items]
Percent of aggregate principal amount	79.30%
Beneficial Owner | Senior Secured Notes due February 15, 2025
Restructuring Cost and Reserve [Line Items]
Percent of aggregate principal amount		70.00%