Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 567,365	$ 528,643	$ 519,508	$ 501,221	$ 518,832	$ 508,676	$ 498,681	$ 482,802	$ 2,116,737	$ 2,008,991	$ 2,093,529
Operating income	45,061	35,874	36,913	25,972	34,956	29,074	29,553	32,487	143,820	126,070	200,456
Net income	$ 141,134	$ 22,517	$ 23,219	$ 16,019	$ 21,811	$ 18,920	$ 18,306	$ 20,092	$ 202,889	$ 79,129	$ 123,714
Basic earnings per share	$ 1.95	$ 0.31	$ 0.32	$ 0.22	$ 0.30	$ 0.26	$ 0.25	$ 0.28	$ 2.81	$ 1.10	$ 1.72
Diluted earnings per share	$ 1.94	$ 0.31	$ 0.32	$ 0.22	$ 0.30	$ 0.26	$ 0.25	$ 0.28	$ 2.80	$ 1.09	$ 1.71