Share-based compensation (Details 4) (USD $)	3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended			6 Months Ended		0 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended		0 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Long-term incentive plan (LTIP)	Jun. 30, 2012 LTIP linked to TRS	Jun. 30, 2013 LTIP linked to TRS	Jun. 30, 2012 LTIP linked to TRS	Mar. 31, 2013 LTIP linked to TRS	Jun. 30, 2013 2011-2013 LTIP linked to TRS	Jun. 30, 2013 2012-2014 LTIP linked to TRS	Feb. 04, 2013 2013-2015 LTIP linked to TRS	Jun. 30, 2013 2013-2015 LTIP linked to TRS	Jun. 30, 2013 LTIP awards linked to other performance conditions	Jun. 30, 2012 LTIP awards linked to other performance conditions	Jun. 30, 2013 LTIP awards linked to other performance conditions	Jun. 30, 2012 LTIP awards linked to other performance conditions	Feb. 04, 2013 2013-2015 LTIP linked to other performance conditions
Share-based compensation
Payout low end of range (as a percent)					0.00%
Payout high end of range (as a percent)					200.00%
Period for calculating total return to shareholders							3 years
Average period used to calculate performance goals										3 years	3 years		3 years
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, beginning of period (in shares)						239,470	239,256	197,385	235,064					341,824	297,602	247,175	182,498
Granted (in shares)						1,442	89,533	78,924							3,600	118,895	118,704
Vested (in shares)							(87,753)	(35,397)								(18,275)
Cancelled (in shares)														(37,351)		(37,351)
Forfeited (in shares)						(1,505)	(5,972)	(1,505)							(6,018)	(5,971)	(6,018)
Outstanding, end of period (in shares)						239,407	235,064	239,407	235,064			89,533		304,473	295,184	304,473	295,184	118,895
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)						$ 29.12	$ 29.12	$ 25.94	$ 32.87					$ 26.00	$ 23.92	$ 25.04	$ 22.63
Granted (in dollars per share)						$ 30.71	$ 32.69	$ 30.71				$ 26.89			$ 26.75	$ 26.89	$ 26.00	$ 26.89
Vested (in dollars per share)							$ 22.45	$ 14.85								$ 18.95
Cancelled (in dollars per share)														$ 24.96		$ 24.96
Forfeited (in dollars per share)						$ 30.39	$ 32.96	$ 30.39							$ 24.23	$ 25.94	$ 24.23
Outstanding, end of period (in dollars per share)						$ 29.12	$ 32.87	$ 29.12	$ 32.87			$ 32.69		$ 26.12	$ 23.95	$ 26.12	$ 23.95	$ 26.89
Total weighted-average grant-date fair value								$ 2,400,000				$ 2,900,000			$ 100,000		$ 3,100,000	$ 3,200,000
Risk-free interest rate (as a percent)							0.38%	0.33%
Expected life							3 years	3 years
Expected volatility (as a percent)							19.40%	25.30%
Range of expected volatility for Peer Group, minimum (as a percent)							12.40%	15.50%
Range of expected volatility for Peer Group, maximum (as a percent)							25.30%	34.50%
Grant date fair value (in dollars per share)						$ 30.71	$ 32.69	$ 30.71
Fair value of vested stock (in dollars)							2,200,000	600,000								600,000
Tax benefits	400,000	600,000	1,100,000	1,200,000			900,000	200,000								200,000
Unrecognized compensation cost							$ 3,600,000							$ 4,500,000		$ 4,500,000
Weighted-average period over which unrecognized compensation cost expected to be recognized							1 year 7 months 6 days									1 year 8 months 12 days