Share-based compensation (Details 5) (LTIP linked to TRS, USD $)	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
LTIP linked to TRS
Share-based compensation
Risk-free interest rate (as a percent)	0.66%	0.38%
Expected life	3 years	3 years
Expected volatility (as a percent)	17.80%	19.40%
Range of expected volatility for Peer Group, minimum (as a percent)	12.40%	12.40%
Range of expected volatility for Peer Group, maximum (as a percent)	23.30%	25.30%
Grant date fair value (in dollars per share)	$ 22.95	$ 32.69