Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Basic and Diluted Earnings per Share [Line Items]
Net income	$ 22,393		$ 19,184
Weighted-average common shares outstanding (in shares)	25,245,497		25,505,962
Weighted-average common share for EPS calculation (in shares)	25,340,669		25,572,541
Basic earnings per common share (in dollars per share)	$ 0.89		$ 0.75
Diluted earnings per common share (in dollars per share)	$ 0.88		$ 0.75
Awards excluded from diluted EPS calculation (in shares)	643,066	[1]	1,085,547	[1]
Restricted Stock Awards [Member]
Basic and Diluted Earnings per Share [Line Items]
Add dilutive effect of share-based awards outstanding (in shares)	79,787		50,206
Options Awards [Member]
Basic and Diluted Earnings per Share [Line Items]
Add dilutive effect of share-based awards outstanding (in shares)	15,385		16,373

[1] Outstanding awards excluded from the diluted earnings per share calculation because the effect of including them would have been anti-dilutive.