Share-based compensation Assumptions to Determine the Fair Value of LTIP Awards Linked to TRS (Details) - LTIP linked to TRS - $ / shares	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate (as a percent)	0.66%	0.38%	0.33%
Expected life	3 years	3 years	3 years
Expected volatility (as a percent)	17.80%	19.40%	25.30%
Range of expected volatility for Peer Group, minimum (as a percent)	12.40%	12.40%	15.50%
Range of expected volatility for Peer Group, maximum (as a percent)	23.30%	25.30%	34.50%
Grant date fair value (in dollars per share)	$ 22.95	$ 32.69	$ 30.71