Description of business and significant accounting policies and practices (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Reportable_Segments	Sep. 30, 2012
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Reportable segments				2
Basic EPS:
Net income	$ 629,000,000		$ 784,000,000	$ 1,651,000,000	$ 1,495,000,000
Less income allocated to RSUs	(11,000,000)		(15,000,000)	(29,000,000)	(27,000,000)
Income allocated to common stock for basic EPS calculation	618,000,000		769,000,000	1,622,000,000	1,468,000,000
Weighted average number of shares outstanding, basic (in shares)	1,096,000,000		1,130,000,000	1,102,000,000	1,138,000,000
Basic EPS (in dollars per share)	$ 0.56		$ 0.68	$ 1.47	$ 1.29
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)	15,000,000		11,000,000	15,000,000	15,000,000
Diluted EPS:
Net income	629,000,000		784,000,000	1,651,000,000	1,495,000,000
Less income allocated to RSUs	(11,000,000)		(14,000,000)	(28,000,000)	(27,000,000)
Income allocated to common stock for diluted EPS calculation	618,000,000		770,000,000	1,623,000,000	1,468,000,000
Weighted average number of shares outstanding, diluted (in shares)	1,111,000,000		1,141,000,000	1,117,000,000	1,153,000,000
Diluted EPS (in dollars per share)	$ 0.56		$ 0.67	$ 1.45	$ 1.27
Antidilutive securities excluded from computation of earnings per share, Amount (in shares)	0		65,000,000	0	65,000,000
Gain (loss) on settlement of interest rate swap		$ 0