Condensed Consolidated Statement of Shareholders' Equity (unaudited) - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Deferred Equity Compensation [Member]
Increase (Decrease) in Shareholders' Equity
Effect of adoption of ASU	$ (32)
Beginning balance (in shares) at Dec. 31, 2017		116,020,852
Beginning balance at Dec. 31, 2017	6,839	$ 1	$ 573	$ 5,892	$ 372	$ 1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	272			272
Dividends (in dollars per share)	(37)			(37)
Common stock repurchases (in shares)		(6,951,126)
Common stock repurchases	(250)		(250)
Share-based compensation and other (in shares)		544,488
Share-based compensation and other	(2)		(2)
Effect on OCI	(188)				(188)
Ending balance (in shares) at Jun. 30, 2018		109,614,214
Ending balance at Jun. 30, 2018	6,634	$ 1	321	6,159	152	1
Beginning balance (in shares) at Mar. 31, 2018		113,709,322
Beginning balance at Mar. 31, 2018	6,784	$ 1	466	6,102	214	1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	75			75
Dividends (in dollars per share)	(18)			(18)
Common stock repurchases (in shares)		(4,163,190)
Common stock repurchases	(152)		(152)
Share-based compensation and other (in shares)		68,082
Share-based compensation and other	7		7
Effect on OCI	(62)				(62)
Ending balance (in shares) at Jun. 30, 2018		109,614,214
Ending balance at Jun. 30, 2018	6,634	$ 1	321	6,159	152	1
Beginning balance (in shares) at Dec. 31, 2018		103,672,592
Beginning balance at Dec. 31, 2018	6,555	$ 1	86	6,374	93	1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	196			196
Dividends (in dollars per share)	(38)			(38)
Common stock repurchases (in shares)		(4,427,735)
Common stock repurchases	(190)		(83)	(107)
Share-based compensation and other (in shares)		556,155
Share-based compensation and other	(3)		(3)
Effect on OCI	202				202
Ending balance (in shares) at Jun. 30, 2019		99,801,012
Ending balance at Jun. 30, 2019	6,722	$ 1	0	6,425	295	1
Beginning balance (in shares) at Mar. 31, 2019		102,270,409
Beginning balance at Mar. 31, 2019	6,669	$ 1	0	6,406	261	1
Increase (Decrease) in Shareholders' Equity
Net income (loss)	142			142
Dividends (in dollars per share)	(19)			(19)
Common stock repurchases (in shares)		(2,519,130)
Common stock repurchases	(111)		(7)	(104)
Share-based compensation and other (in shares)		49,733
Share-based compensation and other	7		7
Effect on OCI	34				34
Ending balance (in shares) at Jun. 30, 2019		99,801,012
Ending balance at Jun. 30, 2019	$ 6,722	$ 1	$ 0	$ 6,425	$ 295	$ 1