Basis of presentation and significant accounting policies and practices - Earnings 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
Basic EPS:
Net income	$ 836	$ 798	$ 696	$ 656	$ 825	$ 826	$ 683	$ 487	$ 2,986	$ 2,821	$ 2,162
Income allocated to RSUs									(43)	(44)	(37)
Income allocated to common stock for basic EPS calculation									$ 2,943	$ 2,777	$ 2,125
Weighted average number of shares outstanding, basic (in shares)									1,030	1,065	1,098
Basic earnings per common share	$ 0.81	$ 0.77	$ 0.66	$ 0.62	$ 0.78	$ 0.77	$ 0.63	$ 0.44	$ 2.86	$ 2.61	$ 1.94
Adjustment for dilutive shares:
Stock-based compensation plans (in shares)									13	15	15
Diluted EPS:
Net income	$ 836	$ 798	$ 696	$ 656	$ 825	$ 826	$ 683	$ 487	$ 2,986	$ 2,821	$ 2,162
Income allocated to RSUs									(42)	(43)	(36)
Income allocated to common stock for diluted EPS									$ 2,944	$ 2,778	$ 2,126
Weighted average number of shares outstanding, diluted (in shares)									1,043	1,080	1,113
Diluted earnings per common share	$ 0.80	$ 0.76	$ 0.65	$ 0.61	$ 0.76	$ 0.76	$ 0.62	$ 0.44	$ 2.82	$ 2.57	$ 1.91