Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, 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
Quarterly Financial Information Disclosure [Abstract]
Total operating revenue	$ 328,268	$ 342,317	$ 346,308	$ 301,495	$ 273,919	$ 288,714	$ 275,891	$ 232,662	$ 1,318,388	$ 1,071,186	$ 781,265
Operating income	31,933	49,681	55,132	37,244	38,023	44,556	34,959	26,844	173,990	144,382	68,873
Net income	$ 19,566	$ 30,884	$ 34,591	$ 23,419	$ 23,991	$ 27,657	$ 16,917	$ 7,883	$ 108,460	$ 76,448	$ 72,481
Net income per share, basic	$ 0.27	$ 0.43	$ 0.48	$ 0.32	$ 0.33	$ 0.38	$ 0.41	$ 0.30	$ 1.50	$ 1.44	$ 2.77
Net income per share, diluted	$ 0.27	$ 0.43	$ 0.48	$ 0.32	$ 0.33	$ 0.38	$ 0.41	$ 0.30	$ 1.49	$ 1.43	$ 2.72