Quarterly Results of Operations (Unaudited) (Details) (USD $) In Thousands, except Per Share data, 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
Quarterly Results of Operations (Unaudited) [Abstract]
Operating revenue	$ 148,259	$ 135,749	$ 132,192	$ 120,201	$ 133,308	$ 121,522	$ 122,132	$ 106,977	$ 536,402	$ 483,939	$ 417,410
Income from operations	23,629	20,320	19,876	13,286	18,677	15,505	13,502	6,055	77,110	53,739	18,550
Net income	$ 14,449	$ 12,912	$ 11,969	$ 7,869	$ 11,817	$ 8,888	$ 7,912	$ 3,419	$ 47,199	$ 32,036	$ 9,802
Net income per share:
Basic	$ 0.51	$ 0.44	$ 0.41	$ 0.27	$ 0.41	$ 0.31	$ 0.27	$ 0.12	$ 1.62	$ 1.11	$ 0.34
Diluted	$ 0.50	$ 0.44	$ 0.40	$ 0.27	$ 0.41	$ 0.31	$ 0.27	$ 0.12	$ 1.60	$ 1.10	$ 0.34