Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ 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	$ 229,607	$ 236,161	$ 216,813	$ 273,960	$ 256,160	$ 231,046	$ 228,114	$ 285,142	$ 956,541	$ 1,000,462	$ 923,038
Operating income	27,809	47,532	25,069	39,549	27,787	37,540	23,730	42,571	139,959	131,628	117,181
Net income	$ 13,625	$ 26,232	$ 12,512	$ 22,267	$ 14,355	$ 20,030	$ 12,022	$ 24,235	$ 74,636	$ 70,642	$ 59,468