Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ 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	$ 237,066	$ 246,144	$ 219,173	$ 254,850	$ 229,607	$ 236,161	$ 216,813	$ 273,960	$ 957,233	$ 956,541	$ 1,000,462
Operating income	30,360	46,342	27,778	35,448	27,809	47,532	25,069	39,549	139,928	139,959	131,628
Net income	$ 14,658	$ 24,221	$ 12,625	$ 17,631	$ 13,625	$ 26,232	$ 12,512	$ 22,267	$ 69,135	$ 74,636	$ 70,642