Basis of presentation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in other income, adoption of accounting guidance		$ 223		$ 669
Increase in income tax expense, adoption of accounting guidance		$ 230		$ 686
Increase in the effective income tax rate		2.00%		2.00%
Tax credits and other tax benefits	$ 398	$ 394	$ 1,200	$ 1,200
Amortization of affordable housing investments	274	$ 268	829	$ 799
Carrying value of investments in affordable housing	7,300		7,300		$ 7,300
Commitments related to affordable housing investments	$ 1,900		$ 1,900		1,800
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle					$ 0	$ (321)