Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Total operating revenue	$ 419,984	$ 456,625	$ 407,339	$ 370,437	$ 328,268	$ 342,317	$ 346,308	$ 301,495	$ 1,654,385	$ 1,318,388	$ 1,071,186
Operating income	68,061	97,804	66,758	49,669	31,933	49,681	55,132	37,244	282,292	173,990	144,382
Net income	$ 43,193	$ 61,103	$ 42,068	$ 30,554	$ 19,566	$ 30,884	$ 34,591	$ 23,419	$ 176,918	$ 108,460	$ 76,448
Basic earnings per share (in dollars per share)	$ 0.59	$ 0.84	$ 0.58	$ 0.42	$ 0.27	$ 0.43	$ 0.48	$ 0.32	$ 2.44	$ 1.50	$ 1.44
Diluted earnings per share (in dollars per share)	$ 0.59	$ 0.84	$ 0.58	$ 0.42	$ 0.27	$ 0.43	$ 0.48	$ 0.32	$ 2.42	$ 1.49	$ 1.43