Employee stock-based incentives - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 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	$ 269	$ 326	$ 856	$ 1,071
Accrual of estimated costs of stock awards to be granted in future periods including those to full-career eligible employees	195	213	683	610
Total noncash compensation expense related to employee stock-based incentive plans	$ 464	$ 539	$ 1,539	$ 1,681