Net Income (loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Income (loss) from continuing operations	$ 558	$ 310	$ 403	$ 521	$ 763	$ 720	$ 638	$ 407	$ 1,792	$ 2,528	$ (1,013)
Income (loss) from discontinued operations, net of income taxes	(2)	0	(1)	(3)	(3)	1	4	0	(6)	2	(4)
Net income (loss)	$ 556	$ 310	$ 402	$ 518	$ 760	$ 721	$ 642	$ 407	$ 1,786	$ 2,530	$ (1,017)
Denominator:
Weighted average shares of common stock - basic	807	830	860	900	945	974	992	1,010	849	980	1,064
Dilutive effect of equity incentive awards (in shares)									7	11	0
Weighted average shares of common stock - diluted	812	837	867	908	950	983	1,004	1,029	856	991	1,064
Income (loss) per share - basic:
Continuing operations (in usd per share)	$ 0.69	$ 0.37	$ 0.47	$ 0.58	$ 0.81	$ 0.74	$ 0.64	$ 0.40	$ 2.11	$ 2.58	$ (0.95)
Discontinued operations (in usd per share)	0	0	0	0	0	0	0	0	(0.01)	0	0
Net income (loss) per share - basic (in usd per share)	0.69	0.37	0.47	0.58	0.81	0.74	0.64	0.40	2.10	2.58	(0.95)
Income (loss) per share - diluted:
Continuing operations (in usd per share)	0.69	0.37	0.46	0.57	0.80	0.73	0.64	0.40	2.10	2.55	(0.95)
Discontinued operations (in usd per share)	0	0	0	0	0	0	0	0	(0.01)	0	0
Net income (loss) per share - diluted (in usd per share)	$ 0.69	$ 0.37	$ 0.46	$ 0.57	$ 0.80	$ 0.73	$ 0.64	$ 0.40	$ 2.09	$ 2.55	$ (0.95)
Common stock equivalents excluded from income per diluted share because their effect would have been anti-dilutive (in shares)									18	12	46