CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions		Common Stock Outstanding [Member]	Preferred Stock [Member]	Additional Paid-in Capital	Retained Earnings		Accumulated Other Comprehensive Income (Loss)	Common Stock in Treasury	Total
Balance, shares at Dec. 31, 2015		390,138,549	1,725,000					47,149
Balance at Dec. 31, 2015		$ 4		$ 3,409	$ (1,082)		$ (48)	$ (1)	$ 2,282
Comprehensive income:
Net income (loss)					(1,132)				(1,132)
Other comprehensive income (loss)							4		4
Comprehensive income (loss)									(1,128)
Stock-based compensation				26					26
Preferred stock dividend				(27)					(27)
Issuance of restricted stock, shares		84,165
Cancellation of restricted stock, shares		(24,333)
Treasury stock, shares								(15,880)
Tax withholding - stock compensation, shares		(524,703)
Tax withholding - stock compensation				(5)					(5)
Balance, shares at Mar. 31, 2016		389,673,678	1,725,000					31,269
Balance at Mar. 31, 2016		$ 4		3,403	(2,214)		(44)	$ (1)	1,148
Balance, shares at Dec. 31, 2015		390,138,549	1,725,000					47,149
Balance at Dec. 31, 2015		$ 4		3,409	(1,082)		(48)	$ (1)	2,282
Comprehensive income:
Net income (loss)									(2,643)
Other comprehensive income (loss)									9
Comprehensive income (loss)									$ (2,634)
Preferred stock dividend, shares									9,917,799
Balance, shares at Dec. 31, 2016		495,248,369	1,725,000					31,269
Balance at Dec. 31, 2016		$ 5		4,677	(3,725)	[1]	(39)	$ (1)	$ 917
Balance, shares at Mar. 31, 2016		389,673,678	1,725,000					31,269
Balance at Mar. 31, 2016		$ 4		3,403	(2,214)		(44)	$ (1)	1,148
Comprehensive income:
Net income (loss)					(593)				(593)
Other comprehensive income (loss)							3		3
Comprehensive income (loss)									(590)
Stock-based compensation				16					16
Preferred stock dividend, shares		3,024,737
Cancellation of restricted stock, shares		(64,762)
Tax withholding - stock compensation, shares		(136,828)
Tax withholding - stock compensation				(1)					(1)
Balance, shares at Jun. 30, 2016		392,496,825	1,725,000					31,269
Balance at Jun. 30, 2016		$ 4		3,418	(2,807)		(41)	$ (1)	573
Comprehensive income:
Net income (loss)					(708)				(708)
Other comprehensive income (loss)							2		2
Comprehensive income (loss)									(706)
Stock-based compensation				8					8
Issuance of common stock, shares		98,900,000
Issuance of common stock		$ 1		1,246					1,247
Preferred stock dividend, shares		2,100,119
Issuance of restricted stock, shares		1,140
Cancellation of restricted stock, shares		(48,534)
Tax withholding - stock compensation				1					1
Tax withholding - stock compensation, shares		(3,179)
Balance, shares at Sep. 30, 2016		493,446,371	1,725,000					31,269
Balance at Sep. 30, 2016		$ 5		4,673	(3,515)		(39)	$ (1)	1,123
Comprehensive income:
Net income (loss)					(210)				(210)
Comprehensive income (loss)									(210)
Stock-based compensation				8					8
Exercise of stock options, shares		44,880
Preferred stock dividend, shares		2,041,533
Issuance of restricted stock, shares		2,167
Cancellation of restricted stock, shares		(27,854)
Tax withholding - stock compensation, shares		(264,542)
Tax withholding - stock compensation				(4)					(4)
Issuance of stock awards, shares		5,814
Balance, shares at Dec. 31, 2016		495,248,369	1,725,000					31,269
Balance at Dec. 31, 2016		$ 5		4,677	(3,725)	[1]	(39)	$ (1)	917
Comprehensive income:
Net income (loss)					351	[1]			351
Comprehensive income (loss)									351
Stock-based compensation				10					10
Preferred stock dividend, shares		2,751,410
Issuance of restricted stock, shares		4,549,122
Cancellation of restricted stock, shares		(113,185)
Performance units vested, shares		121,208
Tax withholding - stock compensation, shares		(59,455)
Balance, shares at Mar. 31, 2017		502,497,469	1,725,000					31,269
Balance at Mar. 31, 2017		$ 5		4,687	(3,374)	[1]	(39)	$ (1)	1,278
Balance, shares at Dec. 31, 2016		495,248,369	1,725,000					31,269
Balance at Dec. 31, 2016		$ 5		4,677	(3,725)	[1]	(39)	$ (1)	$ 917
Comprehensive income:
Preferred stock dividend, shares									10,040,306
Balance, shares at Sep. 30, 2017		509,142,659	1,725,000					31,269
Balance at Sep. 30, 2017		$ 5		4,698	(3,013)	[1]	(37)	$ (1)	$ 1,652
Balance, shares at Dec. 31, 2016		495,248,369	1,725,000					31,269
Balance at Dec. 31, 2016		$ 5		4,677	(3,725)	[1]	(39)	$ (1)	917
Comprehensive income:
Net income (loss)	[2]								1,046
Other comprehensive income (loss)									(5)
Comprehensive income (loss)	[2]								1,041
Balance, shares at Dec. 31, 2017		512,134,311	1,725,000					31,269
Balance at Dec. 31, 2017		$ 5		4,698	(2,679)	[1]	(44)	$ (1)	1,979
Balance, shares at Mar. 31, 2017		502,497,469	1,725,000					31,269
Balance at Mar. 31, 2017		$ 5		4,687	(3,374)	[1]	(39)	$ (1)	1,278
Comprehensive income:
Net income (loss)					284	[1]			284
Other comprehensive income (loss)							1		1
Comprehensive income (loss)									285
Stock-based compensation				10					10
Preferred stock dividend, shares		3,346,865
Issuance of restricted stock, shares		353,803
Cancellation of restricted stock, shares		(303,135)
Tax withholding - stock compensation, shares		(1,729)
Issuance of stock awards, shares		72
Balance, shares at Jun. 30, 2017		505,893,345	1,725,000					31,269
Balance at Jun. 30, 2017		$ 5		4,697	(3,090)	[1]	(38)	$ (1)	1,573
Comprehensive income:
Net income (loss)					77	[1]			77
Other comprehensive income (loss)							1		1
Comprehensive income (loss)									78
Stock-based compensation				9					9
Preferred stock dividend, shares		3,346,738
Preferred stock dividend				(8)					(8)
Issuance of restricted stock, shares		133,197
Cancellation of restricted stock, shares		(192,810)
Tax withholding - stock compensation, shares		(37,811)
Balance, shares at Sep. 30, 2017		509,142,659	1,725,000					31,269
Balance at Sep. 30, 2017		$ 5		4,698	(3,013)	[1]	(37)	$ (1)	1,652
Comprehensive income:
Net income (loss)					334	[1]			334
Other comprehensive income (loss)							(7)		(7)
Comprehensive income (loss)									327
Stock-based compensation				9					9
Preferred stock dividend, shares		3,346,703
Preferred stock dividend				(8)					(8)
Issuance of restricted stock, shares		19,086
Cancellation of restricted stock, shares		(132,898)
Tax withholding - stock compensation, shares		(241,239)
Tax withholding - stock compensation				(1)					(1)
Balance, shares at Dec. 31, 2017		512,134,311	1,725,000					31,269
Balance at Dec. 31, 2017		$ 5		4,698	(2,679)	[1]	(44)	$ (1)	1,979
Comprehensive income:
Net income (loss)					208				208
Other comprehensive income (loss)
Comprehensive income (loss)									208
Stock-based compensation				7					7
Conversion of preferred stock, shares		74,998,614	(1,725,000)
Conversion of preferred stock		$ 1		(1)
Issuance of restricted stock, shares		5,076
Cancellation of restricted stock, shares		(160,168)
Performance units vested, shares		214,866
Tax withholding - stock compensation, shares		(338,808)
Tax withholding - stock compensation				(1)					(1)
Balance, shares at Mar. 31, 2018		586,853,891						31,269
Balance at Mar. 31, 2018		$ 6		4,703	(2,471)		(44)	$ (1)	2,193
Balance, shares at Dec. 31, 2017		512,134,311	1,725,000					31,269
Balance at Dec. 31, 2017		$ 5		4,698	(2,679)	[1]	(44)	$ (1)	1,979
Comprehensive income:
Net income (loss)									537
Other comprehensive income (loss)									8
Comprehensive income (loss)									$ 545
Treasury stock, shares									39,061,269
Treasury stock									$ (180)
Balance, shares at Dec. 31, 2018		585,407,107						39,092,537
Balance at Dec. 31, 2018		$ 6		4,715	(2,142)		(36)	$ (181)	2,362
Balance, shares at Mar. 31, 2018		586,853,891						31,269
Balance at Mar. 31, 2018		$ 6		4,703	(2,471)		(44)	$ (1)	2,193
Comprehensive income:
Net income (loss)					51				51
Other comprehensive income (loss)
Comprehensive income (loss)									51
Stock-based compensation				6					6
Issuance of restricted stock, shares		307,743
Cancellation of restricted stock, shares		(722,465)
Tax withholding - stock compensation, shares		(9,068)
Balance, shares at Jun. 30, 2018		586,430,101						31,269
Balance at Jun. 30, 2018		$ 6		4,709	(2,420)		(44)	$ (1)	2,250
Comprehensive income:
Net income (loss)					(29)				(29)
Other comprehensive income (loss)							4		4
Comprehensive income (loss)									(25)
Stock-based compensation				5					5
Issuance of restricted stock, shares		30,924
Cancellation of restricted stock, shares		(248,342)
Treasury stock, shares								4,829,011
Treasury stock								$ (25)	(25)
Tax withholding - stock compensation, shares		(17,521)
Balance, shares at Sep. 30, 2018		586,195,162						4,860,280
Balance at Sep. 30, 2018		$ 6		4,714	(2,449)		(40)	$ (26)	2,205
Comprehensive income:
Net income (loss)					307				307
Other comprehensive income (loss)							4		4
Comprehensive income (loss)									311
Stock-based compensation				3					3
Issuance of restricted stock, shares		5,819
Cancellation of restricted stock, shares		(673,147)
Treasury stock, shares								34,232,257
Treasury stock								$ (155)	(155)
Tax withholding - stock compensation, shares		(120,727)
Tax withholding - stock compensation				(2)					(2)
Balance, shares at Dec. 31, 2018		585,407,107						39,092,537
Balance at Dec. 31, 2018		$ 6		$ 4,715	$ (2,142)		$ (36)	$ (181)	$ 2,362

[1] Includes a net cumulative-effect adjustment of $59 million related to the recognition of previously unrecognized windfall tax benefits resulting from the adoption of ASU 2016-09 as of the beginning of 2017. This adjustment increased net deferred tax assets and the related income tax valuation allowance by the same amount.

[2] In 2018 and 2017, deferred tax activity incurred in other comprehensive income was offset by a valuation allowance.