Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 474,487	$ 519,769	$ 499,337	$ 437,987	$ 419,984	$ 456,625	$ 407,339	$ 370,437	$ 1,931,580	$ 1,654,385	$ 1,318,388
Operating income	89,975	100,191	105,144	59,953	68,061	97,804	66,758	49,669	355,263	282,292	173,990
Net income	$ 55,909	$ 67,000	$ 64,849	$ 37,706	$ 43,193	$ 61,103	$ 42,068	$ 30,554	$ 225,464	$ 176,918	$ 108,460
Basic earnings per share (in dollars per share)	$ 0.77	$ 0.92	$ 0.89	$ 0.52	$ 0.59	$ 0.84	$ 0.58	$ 0.42	$ 3.10	$ 2.44	$ 1.50
Diluted earnings per share (in dollars per share)	$ 0.76	$ 0.91	$ 0.88	$ 0.51	$ 0.59	$ 0.84	$ 0.58	$ 0.42	$ 3.08	$ 2.42	$ 1.49