Stock-based compensation - Range of exercise prices (Details) (Employee Stock Option, Long-term incentive and director compensation plans, USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 14.47
Range of Exercise Price, Maximum	$ 55.41
Number Outstanding (Shares)	57,647,446	64,930,540
Weighted Average Remaining Contractual Life (Years)	6 years 4 months 24 days
Options Outstanding Weighted Average Exercise Price per Share	$ 32.84	$ 28.98
Number Exercisable (Shares)	27,601,866
Options Exercisable Weighted Average Exercise Price per Share	$ 27.34
Range Of Exercise Prices $14.47 to $20.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 14.47
Range of Exercise Price, Maximum	$ 20.00
Number Outstanding (Shares)	4,061,577
Weighted Average Remaining Contractual Life (Years)	4 years
Options Outstanding Weighted Average Exercise Price per Share	$ 14.98
Number Exercisable (Shares)	4,061,577
Options Exercisable Weighted Average Exercise Price per Share	$ 14.98
Range Of Exercise Prices $20.01 to $30.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 20.01
Range of Exercise Price, Maximum	$ 30.00
Number Outstanding (Shares)	11,270,125
Weighted Average Remaining Contractual Life (Years)	3 years 8 months 12 days
Options Outstanding Weighted Average Exercise Price per Share	$ 25.47
Number Exercisable (Shares)	11,250,200
Options Exercisable Weighted Average Exercise Price per Share	$ 25.46
Range Of Exercise Prices $30.01 to $40.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 30.01
Range of Exercise Price, Maximum	$ 40.00
Number Outstanding (Shares)	28,910,636
Weighted Average Remaining Contractual Life (Years)	6 years 6 months
Options Outstanding Weighted Average Exercise Price per Share	$ 33.00
Number Exercisable (Shares)	12,288,339
Options Exercisable Weighted Average Exercise Price per Share	$ 33.14
Range Of Exercise Prices $40.01 to $50.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 40.01
Range of Exercise Price, Maximum	$ 50.00
Number Outstanding (Shares)	13,399,020
Weighted Average Remaining Contractual Life (Years)	9 years 1 month 6 days
Options Outstanding Weighted Average Exercise Price per Share	$ 44.10
Number Exercisable (Shares)	1,750
Options Exercisable Weighted Average Exercise Price per Share	$ 42.66
Range Of Exercise Prices $50.01 to $55.41
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 50.01
Range of Exercise Price, Maximum	$ 55.41
Number Outstanding (Shares)	6,088
Weighted Average Remaining Contractual Life (Years)	9 years 10 months 24 days
Options Outstanding Weighted Average Exercise Price per Share	$ 55.41
Number Exercisable (Shares)	0