Business changes and developments (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended			9 Months Ended
	Apr. 02, 2015	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Class of Stock [Line Items]
Effective tax rate		(1.10%)		29.70%	18.50%	30.50%
Noncash tax benefit due to resolution of tax audits		$ 2,200			$ 2,700
Reduced effective tax rate due to resolution of tax audits		(32.00%)			(11.70%)
Decrease in unrecognized tax benefits					$ 2,000
Trust preferred securities redeemed	$ 1,500
Common stock, dividends, per share (in dollars per share)					$ 1.28	$ 1.18
Common stock
Class of Stock [Line Items]
Common stock, dividends, per share (in dollars per share)		$ 0.44	$ 0.40
Preferred stock
Class of Stock [Line Items]
Issuance of preferred stock		$ 1,200			$ 6,005	$ 8,905