Share-based compensation (Details 2) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Share-based compensation
Share-based compensation expense (in dollars)	$ 1,700,000	$ 500,000	$ 3,500,000	$ 1,700,000
Income tax benefit (in dollars)	600,000	100,000	1,200,000	500,000
Nonqualified stock options (NQSOs)
Share-based compensation
Outstanding shares (in shares)					55,500
Weighted-average exercise price (in dollars per share)					$ 20.92
Aggregate intrinsic value (in dollars)	200,000		200,000
Shares exercised	21,500	69,500	33,500	102,000
Weighted-average exercise price (in dollars per share)	$ 20.93	$ 21.07	$ 21.20	$ 20.82
Cash received from exercise (in dollars)	450,000	1,465,000	710,000	2,123,000
Intrinsic value of shares exercised (in dollars)	174,000	581,000	265,000	840,000
Tax benefit realized for the deduction of exercises (in dollars)	68,000	170,000	103,000	271,000
Nonqualified stock options (NQSOs) | 2003
Share-based compensation
Range of exercise prices (in dollars per share)			$ 20.49
Number of options (in shares)	22,000		22,000
Weighted-average remaining contractual life (in years)	0.7		0.7
Weighted-average exercise price of options outstanding and exercisable (in dollars per share)	$ 20.49		$ 20.49
Stock appreciation rights (SARs)
Share-based compensation
Lower range of exercise prices (in dollars per share)			$ 26.02
Upper range of exercise prices (in dollars per share)			$ 26.18
Number of options (in shares)	170,000		170,000
Weighted-average remaining contractual life (in years)	2.4		2.4
Weighted-average exercise price of shares outstanding and exercisable (in dollars per share)	$ 26.12		$ 26.12
Outstanding shares (in shares)					282,000
Weighted-average exercise price (in dollars per share)					$ 26.14
Aggregate intrinsic value (in dollars)	500,000		500,000
Shares underlying SARS expired (in shares)		4,000		40,000
Weighted-average price of shares expired (in dollars per share)		$ 26.18		$ 26.11
Shares exercised	112,000		112,000
Intrinsic value of shares exercised	194,000		194,000
Tax benefit realized for the deduction of exercises	$ 76,000		$ 76,000
Stock appreciation rights (SARs) | 2004
Share-based compensation
Range of exercise prices (in dollars per share)			$ 26.02
Number of options (in shares)	62,000		62,000
Weighted-average remaining contractual life (in years)	1.8		1.8
Weighted-average exercise price of shares outstanding and exercisable (in dollars per share)	$ 26.02		$ 26.02
Stock appreciation rights (SARs) | 2005
Share-based compensation
Range of exercise prices (in dollars per share)			$ 26.18
Number of options (in shares)	108,000		108,000
Weighted-average remaining contractual life (in years)	2.7		2.7
Weighted-average exercise price of shares outstanding and exercisable (in dollars per share)	$ 26.18		$ 26.18