Share-based compensation (Details 2) (USD $)	3 Months Ended		9 Months Ended		0 Months Ended		3 Months Ended				9 Months Ended					3 Months Ended				9 Months Ended					9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 28, 2013 Stock award	Jun. 29, 2012 Stock award	Sep. 30, 2013 Nonqualified stock options (NQSOs)		Sep. 30, 2012 Nonqualified stock options (NQSOs)		Sep. 30, 2013 Nonqualified stock options (NQSOs)		Sep. 30, 2012 Nonqualified stock options (NQSOs)		Dec. 31, 2012 Nonqualified stock options (NQSOs)	Sep. 30, 2013 Stock appreciation rights (SARs)		Sep. 30, 2012 Stock appreciation rights (SARs)		Sep. 30, 2013 Stock appreciation rights (SARs)		Sep. 30, 2012 Stock appreciation rights (SARs)		Dec. 31, 2012 Stock appreciation rights (SARs)	Sep. 30, 2013 Stock appreciation rights (SARs) 2004	Sep. 30, 2013 Stock appreciation rights (SARs) 2005
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based compensation expense (in dollars)	$ 2,500,000	$ 1,400,000	$ 6,000,000	$ 5,400,000
Share-based compensation
Number of shares granted					33,184	29,448
Fair value of vested stock (in dollars)					800,000	800,000
Income tax benefit (in dollars)	900,000	500,000	2,200,000	1,900,000	300,000	300,000
Lower range of exercise prices (in dollars per share)																				$ 26.02
Upper range of exercise prices (in dollars per share)																				$ 26.18
Range of exercise prices (in dollars per share)																									$ 26.02	$ 26.18
Number of shares underlying SARs																164,000				164,000					62,000	102,000
Weighted-average remaining contractual life																				1 year 2 months 12 days					7 months 6 days	1 year 6 months
Weighted-average exercise price of shares outstanding and exercisable (in dollars per share)																$ 26.12				$ 26.12					$ 26.02	$ 26.18
Outstanding shares (in shares)							0				0				14,000									164,000
Weighted-average exercise price (in dollars per share)															$ 20.49									$ 26.12
Aggregate intrinsic value (in dollars)																0				0
Shares exercised							0		8,000		14,000		41,500			0		2,000		0		114,000
Weighted-average exercise price (in dollars per share)							$ 0.00		$ 20.49		$ 20.49		$ 21.06
Cash received from exercise (in dollars)							0		164,000		287,000		874,000
Weighted-average price of shares exercised (in dollars per share)																$ 0.00		$ 26.18		$ 0.00		$ 26.17
Intrinsic value of shares exercised (in dollars)							0	[1]	89,000	[1]	128,000	[1]	354,000	[1]		0	[2]	3,000	[2]	0	[2]	197,000	[2]
Tax benefit realized for the deduction of exercises (in dollars)							$ 0		$ 35,000		$ 50,000		$ 138,000			$ 0		$ 1,000		$ 0		$ 77,000

[1] Intrinsic value is the amount by which the fair market value of the underlying stock and the related dividend equivalents exceeds the exercise price of the option.

[2] Intrinsic value is the amount by which the fair market value of the underlying stock and the related dividend equivalent rights exceeds the exercise price of the right.