Net Income Per Share (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net Income Per Share [Abstract]
Net income	$ 9,445	$ 11,452	$ 5,621	$ 13,884	$ 10,844	$ 4,099	[1]	$ 6,741	$ 8,786	$ 40,402	$ 30,470	$ 22,440
Less distributed and undistributed earnings allocated to participating securities										(1,224)	(908)	(657)
Net income allocated to common shares										$ 39,178	$ 29,562	$ 21,783
Weighted average common shares outstanding (in shares)										25,007	25,405	25,254
Incremental shares from assumed conversions of common stock options (in shares)										60	92	114
Adjusted weighted average common shares outstanding (in shares)										25,067	25,497	25,368
Potentially dilutive securities not included in weighted average share calculation due to anti-dilutive effect (in shares)										29	29	372

[1] Included in the results for the third quarter of 2011 is a $4.4 million loss related to the exchange of an aircraft and a $3.1 million loss related to a settlement with the State of California. Please read Note 13, "Commitments and Contingencies," for additional information on the settlement with the State of California.