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	$ 426,509	$ 551,623	$ 479,796	$ 460,072	$ 464,749	$ 554,926	$ 440,445	$ 390,839
Operating income	41,498	122,407	74,489	58,415	62,964	122,362	68,386	53,569	$ 296,809	$ 307,281	$ 274,465
Net income	$ 31,015	$ 67,781	$ 35,362	$ 25,055	$ 29,077	$ 68,346	$ 32,211	$ 22,523	$ 159,213	$ 152,157	$ 127,263