Net Income per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income	$ 1,023	$ 673	$ 676	$ (2,326)	$ 850	$ 689	$ 640	$ 677	$ 46	$ 2,856	$ 2,609
Weighted average shares of common stock - basic	1,230	1,242	1,258	1,276	1,295	1,295	1,297	1,295	1,251	1,295	1,292
Dilutive effect of equity incentive awards									11	18	21
Weighted average shares of common stock - diluted	1,241	1,251	1,267	1,276	1,310	1,310	1,313	1,319	1,262	1,313	1,313
Basic (usd per share)	$ 0.83	$ 0.54	$ 0.54	$ (1.82)	$ 0.66	$ 0.53	$ 0.49	$ 0.52	$ 0.04	$ 2.20	$ 2.02
Diluted (usd per share)	$ 0.82	$ 0.54	$ 0.53	$ (1.82)	$ 0.65	$ 0.53	$ 0.49	$ 0.51	$ 0.04	$ 2.18	$ 1.99
Common stock equivalents excluded from income per diluted share because their effect would have been anti-dilutive									7	4	4