Share-based compensation (Details) (USD $)	3 Months Ended		6 Months Ended		1 Months Ended			6 Months Ended		6 Months Ended						6 Months Ended							6 Months Ended									0 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Apr. 30, 2014 Stock appreciation rights (SARs)	Jun. 30, 2014 Stock appreciation rights (SARs)	Dec. 31, 2013 Stock appreciation rights (SARs)	Jun. 30, 2014 Restricted shares	Dec. 31, 2013 Restricted shares	Jun. 30, 2014 Restricted stock units	Jun. 30, 2013 Restricted stock units	Mar. 31, 2014 Restricted stock units	Dec. 31, 2013 Restricted stock units	Mar. 31, 2013 Restricted stock units	Dec. 31, 2012 Restricted stock units	Jun. 30, 2014 Long-term incentive plan (LTIP)	Jun. 30, 2014 LTIP linked to TRS	Jun. 30, 2013 LTIP linked to TRS	Mar. 31, 2014 LTIP linked to TRS	Dec. 31, 2013 LTIP linked to TRS	Mar. 31, 2013 LTIP linked to TRS	Dec. 31, 2012 LTIP linked to TRS	Jun. 30, 2014 2012-2014 LTIP linked to TRS	Jun. 30, 2014 2013-2015 LTIP linked to TRS	Jun. 30, 2014 2014 to 2016 LTIP linked to TRS	Jun. 30, 2014 LTIP awards linked to other performance conditions	Jun. 30, 2013 LTIP awards linked to other performance conditions	Mar. 31, 2014 LTIP awards linked to other performance conditions	Dec. 31, 2013 LTIP awards linked to other performance conditions	Mar. 31, 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)	Jun. 30, 2014 Equity and Incentive Plan (EIP)	Jun. 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
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	264,326	300,313	332,158	288,151	301,145	315,094		257,956	235,064	258,243	232,127	235,064	239,256				359,782	304,473	360,070	296,843	341,824	247,175
Outstanding, weighted-average exercise price								$ 22.21	$ 22.21	$ 25.74	$ 25.15	$ 25.04	$ 25.17	$ 25.15	$ 22.82		$ 28.45	$ 32.87	$ 28.46	$ 32.88	$ 32.87	$ 29.12				$ 26.01	$ 26.12	$ 26.01	$ 26.14	$ 26.00	$ 25.04
Unrecognized compensation cost								46,000		5,600,000							3,300,000									4,800,000
Weighted-average period over which unrecognized compensation cost expected to be recognized								4 months 24 days		2 years 9 months 18 days							1 year 7 months 6 days									1 year 6 months
Fair value of vested stock (in dollars)										4,000,000	3,500,000							2,200,000								1,900,000	600,000
Income tax benefit (in dollars)	$ 1,100,000	$ 500,000	$ 1,900,000	$ 1,300,000						$ 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