Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 519,846	$ 574,841	$ 553,421	$ 493,355	$ 474,487	$ 519,769	$ 499,337	$ 437,987	$ 2,141,463	$ 1,931,580	$ 1,654,385
Operating income	120,337	157,219	122,315	109,251	89,975	100,191	105,144	59,953	509,122	355,263	282,292
Net income	$ 74,400	$ 97,114	$ 76,704	$ 69,002	$ 55,909	$ 67,000	$ 64,849	$ 37,706	$ 317,220	$ 225,464	$ 176,918
Basic earnings per share (in dollars per share)	$ 1.04	$ 1.35	$ 1.06	$ 0.94	$ 0.77	$ 0.92	$ 0.89	$ 0.52	$ 4.39	$ 3.10	$ 2.44
Diluted earnings per share (in dollars per share)	$ 1.04	$ 1.35	$ 1.05	$ 0.94	$ 0.76	$ 0.91	$ 0.88	$ 0.51	$ 4.38	$ 3.08	$ 2.42