Summary of Significant Accounting Policies (Details 5) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator:
Net income	$ 394	$ 723	$ 423	$ 766	$ 712	$ 175	$ (624)	$ 524	$ 2,306	$ 787	$ 911
Denominator:
Weighted average common shares outstanding									489.4	520.7	540.3
Effect of dilutive potential common shares:
Stock options (in shares)									2.4	1.8	2.0
Restricted stock units and performance stock awards (non-participating) (in shares)									1.2	0.6	0.2
Weighted average common and dilutive potential common shares outstanding									493.0	523.1	542.5
Earnings per share - Basic (in dollars per share)	$ 0.82	$ 1.49	$ 0.86	$ 1.54	$ 1.41	$ 0.34	$ (1.19)	$ 0.99	$ 4.71	$ 1.51	$ 1.69
Earnings per share - Diluted (in dollars per share)	$ 0.81	$ 1.48	$ 0.86	$ 1.53	$ 1.40	$ 0.34	$ (1.19)	$ 0.98	$ 4.68	$ 1.50	$ 1.68
Other Earnings Per Share Disclosures
Antidilutive stock options, exercise price exceeds market price (in shares)									20.4	27.2	26.7
Antidilutive stock options, exercise price exceeds market price, exercise price range, low end of range (in dollars per share)									$ 26.56	$ 22.71	$ 27.36
Antidilutive stock options, exercise price exceeds market price, exercise price range, high end of range (in dollars per share)									$ 62.84	$ 62.84	$ 64.53