Selected Unaudited Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total operating revenues	$ 249,307	$ 240,448	$ 278,326	$ 340,514	$ 245,108	$ 236,333	$ 263,792	$ 320,342	$ 1,108,595	$ 1,065,575	$ 901,123
Total operating expenses	249,550	242,635	234,363	259,129	229,712	235,662	231,168	238,618	985,677	935,160	791,827
Operating income	(243)	(2,187)	43,963	81,385	15,396	671	32,624	81,724	122,918	130,415	109,296
Interest expense, net	89	112	165	1,471	3,067	991	1,280	1,260	1,837	6,598	5,838
Income before income taxes	(332)	(2,299)	43,798	79,914	12,329	(320)	31,344	80,464	121,081	123,817	103,458
Income tax expense	(2,025)	(1,768)	9,106	15,871	(1,943)	(4,893)	1,517	10,433	21,184	5,114	17,571
Net income	$ 1,693	$ (531)	$ 34,692	$ 64,043	$ 14,272	$ 4,573	$ 29,827	$ 70,031	$ 99,897	$ 118,703	$ 85,887
Basic earnings per common share (in USD per share)	$ 0.03	$ (0.01)	$ 0.66	$ 1.21	$ 0.27	$ 0.09	$ 0.57	$ 1.36	$ 1.91	$ 2.27	$ 1.70
Diluted earnings per common share (in USD per share)	$ 0.03	$ (0.01)	$ 0.64	$ 1.17	$ 0.26	$ 0.08	$ 0.55	$ 1.29	$ 1.88	$ 2.18	$ 1.61