Net Income (loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Income (loss) from continuing operations	$ 523	$ 545	$ 430	$ 449	$ 729	$ 509	$ 458	$ (2,561)	$ 1,947	$ (865)	$ 2,067
Income (loss) from discontinued operations, net of income taxes	(46)	(6)	(347)	177	294	164	218	235	(222)	911	789
Net income	$ 477	$ 539	$ 83	$ 626	$ 1,023	$ 673	$ 676	$ (2,326)	$ 1,725	$ 46	$ 2,856
Denominator:
Weighted average shares of common stock - basic	1,191	1,210	1,217	1,216	1,230	1,242	1,258	1,276	1,208	1,251	1,295
Dilutive effect of equity incentive awards (in shares)									12	0	18
Weighted average shares of common stock - diluted	1,204	1,223	1,225	1,229	1,241	1,251	1,267	1,276	1,220	1,251	1,313
Income (loss) per share - basic:
Continuing operations (in usd per share)	$ 0.44	$ 0.45	$ 0.35	$ 0.37	$ 0.59	$ 0.41	$ 0.36	$ (2.01)	$ 1.61	$ (0.69)	$ 1.60
Discontinued operations (in usd per share)	(0.04)	0.00	(0.28)	0.14	0.24	0.13	0.18	0.19	(0.18)	0.73	0.60
Net income per share - basic (in usd per share)	0.40	0.45	0.07	0.51	0.83	0.54	0.54	(1.82)	1.43	0.04	2.20
Income (loss) per share - diluted:
Continuing operations (in usd per share)	0.43	0.45	0.35	0.37	0.59	0.41	0.36	(2.01)	1.60	(0.69)	1.58
Discontinued operations (in usd per share)	(0.04)	0.00	(0.28)	0.14	0.23	0.13	0.17	0.19	(0.18)	0.73	0.60
Net income per share - diluted (in usd per share)	$ 0.39	$ 0.45	$ 0.07	$ 0.51	$ 0.82	$ 0.54	$ 0.53	$ (1.82)	$ 1.42	$ 0.04	$ 2.18
Common stock equivalents excluded from income per diluted share because their effect would have been anti-dilutive (in shares)									2	54	4