Net Income Per Share (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net Income Per Share [Abstract]
Net income	$ 10,844	$ 4,099	$ 6,741	$ 8,786	$ 7,789	$ 7,234	$ 5,118	$ 2,299	$ 30,470	$ 22,440	$ 16,574
Less income allocated to participating securities									(908)	(657)	(462)
Net income allocated to common shares									$ 29,562	$ 21,783	$ 16,112
Weighted average common shares outstanding (in shares)									25,405	25,254	24,768
Incremental shares from assumed conversions of common stock options (in shares)									92	114	148
Adjusted weighted average common shares outstanding (in shares)									25,497	25,368	24,916
Potentially dilutive securities not included in weighted average share calculation due to anti-dilutive effect (in shares)									29	372	541