Employee stock-based incentives - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Cost of prior grants of restricted stock units (“RSUs”) and stock appreciation rights (“SARs”) that are amortized over their applicable vesting periods	$ 295	$ 335	$ 587	$ 745
Accrual of estimated costs of stock awards to be granted in future periods including those to full-career eligible employees	215	189	488	397
Total noncash compensation expense related to employee stock-based incentive plans	$ 510	$ 524	$ 1,075	$ 1,142