Share-based compensation (Details) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended			9 Months Ended		9 Months Ended						9 Months Ended							9 Months Ended									0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Apr. 30, 2014 Stock appreciation rights (SARs)	Sep. 30, 2014 Stock appreciation rights (SARs)	Dec. 31, 2013 Stock appreciation rights (SARs)	Sep. 30, 2014 Restricted shares	Dec. 31, 2013 Restricted shares	Sep. 30, 2014 Restricted stock units	Sep. 30, 2013 Restricted stock units	Jun. 30, 2014 Restricted stock units	Dec. 31, 2013 Restricted stock units	Jun. 30, 2013 Restricted stock units	Dec. 31, 2012 Restricted stock units	Sep. 30, 2014 Long-term incentive plan (LTIP)	Sep. 30, 2014 LTIP linked to TRS	Sep. 30, 2013 LTIP linked to TRS	Jun. 30, 2014 LTIP linked to TRS	Dec. 31, 2013 LTIP linked to TRS	Jun. 30, 2013 LTIP linked to TRS	Dec. 31, 2012 LTIP linked to TRS	Sep. 30, 2014 2012-2014 LTIP linked to TRS	Sep. 30, 2014 2013-2015 LTIP linked to TRS	Sep. 30, 2014 2014 to 2016 LTIP linked to TRS	Sep. 30, 2014 LTIP awards linked to other performance conditions	Sep. 30, 2013 LTIP awards linked to other performance conditions	Jun. 30, 2014 LTIP awards linked to other performance conditions	Dec. 31, 2013 LTIP awards linked to other performance conditions	Jun. 30, 2013 LTIP awards linked to other performance conditions	Dec. 31, 2012 LTIP awards linked to other performance conditions	Mar. 01, 2014 Equity and Incentive Plan (EIP)	Sep. 30, 2014 Equity and Incentive Plan (EIP)	Sep. 30, 2014 Stock Option and Incentive Plan [Member]	Sep. 30, 2014 Nonemployee Director Stock Plan (2011 Director Plan)
Share-based compensation
Additional shares available for issuance																																1,500,000
Shares remaining available for future issuance																																	3,600,000
Shares that can be issued upon vesting of outstanding units and achievement of performance goals																																	1,100,000
Estimated possible future issuances of shares																																		1,000
Number of shares available for grant																																			169,290
Outstanding awards (in shares)							164,000
Weighted-average exercise price (in dollars per share)							$ 26.12
Aggregate intrinsic value (in dollars)						$ 0
Number of shares underlying SARs					62,000
Weighted-average exercise price (in dollars per share)					$ 26.02
Outstanding awards (in shares)								4,503	4,503	263,576	290,492	264,326	288,151	300,313	315,094		257,956	232,127	257,956	232,127	235,064	239,256				359,782	301,096	359,782	296,843	304,473	247,175
Outstanding, weighted-average exercise price								$ 22.21	$ 22.21	$ 25.76	$ 25.16	$ 25.74	$ 25.17	$ 25.15	$ 22.82		$ 28.45	$ 32.88	$ 28.45	$ 32.88	$ 32.87	$ 29.12				$ 26.01	$ 26.12	$ 26.01	$ 26.14	$ 26.12	$ 25.04
Unrecognized compensation cost								21,000		5,100,000							2,700,000									4,000,000
Weighted-average period over which unrecognized compensation cost expected to be recognized								2 months 12 days		2 years 7 months 6 days							1 year 3 months 18 days									1 year 3 months 18 days
Fair value of vested stock (in dollars)										4,100,000	3,600,000							2,200,000								1,900,000	600,000
Income tax benefit (in dollars)	$ 700,000	$ 900,000	$ 2,600,000	$ 2,200,000						$ 1,300,000	$ 1,000,000							$ 900,000								$ 800,000	$ 200,000
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
Award requisite service period																							3 years	3 years	3 years