Basis of presentation and significant accounting policies and practices - Earnings per share (Details) (USD $) In Millions, except 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
Basic EPS:
Net income	$ 825	$ 826	$ 683	$ 487	$ 511	$ 629	$ 660	$ 362	$ 2,821	$ 2,162	$ 1,759
Income allocated to RSUs									(44)	(37)	(31)
Income allocated to common stock for basic EPS calculation									2,777	2,125	1,728
Weighted average number of shares outstanding, basic (in shares)									1,065,000,000	1,098,000,000	1,132,000,000
Basic earnings per common share	$ 0.78	$ 0.77	$ 0.63	$ 0.44	$ 0.46	$ 0.56	$ 0.59	$ 0.32	$ 2.61	$ 1.94	$ 1.53
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)									15,000,000	15,000,000	14,000,000
Diluted EPS:
Net income	825	826	683	487	511	629	660	362	2,821	2,162	1,759
Income allocated to RSUs									(43)	(36)	(31)
Income allocated to common stock for diluted EPS calculation									$ 2,778	$ 2,126	$ 1,728
Weighted average number of shares outstanding, diluted (in shares)									1,080,000,000	1,113,000,000	1,146,000,000
Diluted earnings per common share	$ 0.76	$ 0.76	$ 0.62	$ 0.44	$ 0.46	$ 0.56	$ 0.58	$ 0.32	$ 2.57	$ 1.91	$ 1.51
Antidilutive securities excluded from computation of earnings per share, Amount (in shares)									11,000,000	0	52,000,000