Quarterly Results of Operations (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 revenues	$ 528,783	$ 534,448	$ 534,644	$ 495,654	$ 553,186	$ 551,961	$ 542,120	$ 492,022	$ 2,093,529	$ 2,139,289	$ 2,029,183
Operating income	57,261	52,800	52,210	38,185	52,627	41,690	42,330	23,441	200,456	160,088	139,726
Net Income	$ 36,648	$ 32,076	$ 31,848	$ 23,142	$ 32,709	$ 25,970	$ 25,632	$ 14,339	$ 123,714	$ 98,650	$ 86,785
Basic earnings per share	$ 0.51	$ 0.45	$ 0.44	$ 0.32	$ 0.45	$ 0.36	$ 0.36	$ 0.20	$ 1.72	$ 1.37	$ 1.19
Diluted earnings per share	$ 0.51	$ 0.44	$ 0.44	$ 0.32	$ 0.45	$ 0.36	$ 0.35	$ 0.20	$ 1.71	$ 1.36	$ 1.18