Net Income per Share (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Net Income per Share [Abstract]
Net income	$ 13,173	$ 13,884
Less distributed and undistributed earnings allocated to participating securities	(383)	(402)
Net income allocated to common shares	$ 12,790	$ 13,482
Weighted average common shares outstanding (in shares)	24,897	25,087
Incremental shares from assumed conversions of common stock options (in shares)	30	71
Adjusted weighted average common shares outstanding (in shares)	24,927	25,158
Potentially dilutive securities not included in weighted average share calculation due to anti-dilutive effect (in shares)	16	16