Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 518,832	$ 508,676	$ 498,681	$ 482,802	$ 528,783	$ 534,448	$ 534,644	$ 495,654	$ 2,008,991	$ 2,093,529	$ 2,139,289
Operating income	34,956	29,074	29,553	32,487	57,261	52,800	52,210	38,185	126,070	200,456	160,088
Net Income	$ 21,811	$ 18,920	$ 18,306	$ 20,092	$ 36,648	$ 32,076	$ 31,848	$ 23,142	$ 79,129	$ 123,714	$ 98,650
Basic earnings per share	$ 0.30	$ 0.26	$ 0.25	$ 0.28	$ 0.51	$ 0.45	$ 0.44	$ 0.32	$ 1.10	$ 1.72	$ 1.37
Diluted earnings per share	$ 0.30	$ 0.26	$ 0.25	$ 0.28	$ 0.51	$ 0.44	$ 0.44	$ 0.32	$ 1.09	$ 1.71	$ 1.36