Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 646,365	$ 629,735	$ 619,130	$ 562,684	$ 567,365	$ 528,643	$ 519,508	$ 501,221	$ 2,457,914	$ 2,116,737	$ 2,008,991
Operating income	74,931	63,386	50,783	35,115	45,061	35,874	36,913	25,972	224,215	143,820	126,070
Net income	$ 54,563	$ 47,514	$ 38,264	$ 27,807	$ 141,134	$ 22,517	$ 23,219	$ 16,019	$ 168,148	$ 202,889	$ 79,129
Basic earnings per share	$ 0.77	$ 0.67	$ 0.53	$ 0.38	$ 1.95	$ 0.31	$ 0.32	$ 0.22	$ 2.35	$ 2.81	$ 1.10
Diluted earnings per share	$ 0.77	$ 0.66	$ 0.53	$ 0.38	$ 1.94	$ 0.31	$ 0.32	$ 0.22	$ 2.33	$ 2.80	$ 1.09