Net Income Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Earnings Per Share [Abstract]
Net income	$ 13,884	$ 8,786
Less income allocated to participating securities	(402)	(264)
Net income allocated to common shares	$ 13,482	$ 8,522
Weighted average common shares outstanding	25,087	25,488
Incremental shares from assumed conversions of common stock options	71	119
Adjusted weighted average common shares outstanding	25,158	25,607
Potentially dilutive securities not included in weighted average share calculation due to anti-dilutive effect	16	0