Stock-based compensation - Options vested and expected to vest (Details) (Long-term incentive and director compensation plans, Employee Stock Option, USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Long-term incentive and director compensation plans | Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of outstanding shares, vested and expected to vest (in shares)	56,328,323	[1]
Weighted-average remaining contractual life, vested and expected to vest (in years)	6 years 3 months 18 days	[1]
Weighted-average exercise price per share, vested and expected to vest (in dollars per share)	$ 32.69	[1]
Intrinsic value, vested and expected to vest (millions of dollars)	$ 1,170	[1]
Outstanding options exercisable (in shares)	27,601,866
Weighted-average remaining contractual life, options exercisable	4 years 4 months 24 days
Weighted-average exercise price per share, options exercisable (in dollars per share)	$ 27.34
Intrinsic value, options exercisable (millions of dollars)	721
Expected forfeitures, effects of which are included (in shares)	1,000,000
Aggregate intrinsic value of stock options outstanding, excluding the effects of expected forfeitures	$ 1,189

[1] Includes effects of expected forfeitures of approximately 1 million shares. Excluding the effects of expected forfeitures, the aggregate intrinsic value of stock options outstanding was $1,189 million.