Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ 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	$ 262,983	$ 247,511	$ 230,026	$ 264,931	$ 237,066	$ 246,144	$ 219,173	$ 254,850	$ 1,005,451	$ 957,233	$ 956,541
Operating income	37,994	38,392	29,067	42,775	30,360	46,342	27,778	35,448	148,228	139,928	139,959
Net income	$ 20,431	$ 22,325	$ 14,241	$ 22,419	$ 14,658	$ 24,221	$ 12,625	$ 17,631	$ 79,416	$ 69,135	$ 74,636