Share-based compensation (Details 4) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Weighted-average grant-date fair value per share
Total weighted-average grant-date fair value	$ 0	$ 0
Tax benefits realized for the tax deductions	1,100,000	500,000	1,900,000	1,300,000
Restricted shares
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)			4,503
Outstanding, end of period (in shares)	4,503		4,503
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)			$ 22.21
Outstanding, end of period (in dollars per share)	$ 22.21		$ 22.21
Unrecognized compensation cost	46,000		46,000
Weighted-average period over which unrecognized compensation cost expected to be recognized			4 months 24 days
Restricted stock units
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	332,158	301,145	288,151	315,094
Granted (in shares)			115,036	107,231
Vested (in shares)	(67,832)	(832)	(138,861)	(114,044)
Forfeited (in shares)				(7,968)
Outstanding, end of period (in shares)	264,326	300,313	264,326	300,313
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 25.04	$ 25.15	$ 25.17	$ 22.82
Granted (in dollars per share)			$ 25.19	$ 26.89
Vested (in dollars per share)	$ 22.31	$ 26.60	$ 24.09	$ 20.34
Forfeited (in dollars per share)				$ 25.26
Outstanding, end of period (in dollars per share)	$ 25.74	$ 25.15	$ 25.74	$ 25.15
Total weighted-average grant-date fair value			2,900,000	2,900,000
Unrecognized compensation cost	5,600,000		5,600,000
Fair value of vested stock (in dollars)			4,000,000	3,500,000
Weighted-average period over which unrecognized compensation cost expected to be recognized			2 years 9 months 18 days
Tax benefits realized for the tax deductions			1,300,000	1,000,000
LTIP linked to TRS
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	258,243	235,064	232,127	239,256
Granted (in shares)	731		97,524	89,533
Vested (in shares)			(70,189)	(87,753)
Forfeited (in shares)	(1,018)		(1,506)	(5,972)
Outstanding, end of period (in shares)	257,956	235,064	257,956	235,064
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 28.46	$ 32.87	$ 32.88	$ 29.12
Granted (in dollars per share)	$ 22.95		$ 22.95	$ 32.69
Vested (in dollars per share)			$ 35.46	$ 22.45
Forfeited (in dollars per share)	$ 26.50		$ 28.32	$ 32.96
Outstanding, end of period (in dollars per share)	$ 28.45	$ 32.87	$ 28.45	$ 32.87
Total weighted-average grant-date fair value	0	0	2,200,000	2,900,000
Unrecognized compensation cost	3,300,000		3,300,000
Fair value of vested stock (in dollars)				2,200,000
Weighted-average period over which unrecognized compensation cost expected to be recognized			1 year 7 months 6 days
Tax benefits realized for the tax deductions				900,000
LTIP awards linked to other performance conditions
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	360,070	341,824	296,843	247,175
Granted (in shares)	730		129,603	118,895
Vested (in shares)			(65,089)	(18,275)
Cancelled (shares)		(37,351)		(37,351)
Forfeited (in shares)	(1,018)		(1,575)	(5,971)
Outstanding, end of period (in shares)	359,782	304,473	359,782	304,473
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 26.01	$ 26.00	$ 26.14	$ 25.04
Granted (in dollars per share)	$ 23.34		$ 25.18	$ 26.89
Vested (in dollars per share)			$ 24.95	$ 18.95
Cancelled (in dollars per share)		$ 24.96		$ 24.96
Forfeited (in dollars per share)	$ 25.81		$ 26.07	$ 25.94
Outstanding, end of period (in dollars per share)	$ 26.01	$ 26.12	$ 26.01	$ 26.12
Total weighted-average grant-date fair value	0	0	3,300,000	3,200,000
Unrecognized compensation cost	4,800,000		4,800,000
Fair value of vested stock (in dollars)			1,900,000	600,000
Weighted-average period over which unrecognized compensation cost expected to be recognized			1 year 6 months
Tax benefits realized for the tax deductions			$ 800,000	$ 200,000