Summary of Quarterly Operating Results (Unaudited) - Summary of Quarterly Operating Results (Unaudited) (Detail) - USD ($) $ / shares in Units, $ 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
Income Statement [Abstract]
Operating revenue	$ 353,251	$ 350,062	$ 358,160	$ 317,037	$ 300,223	$ 316,442	$ 311,905	$ 289,911	$ 1,378,510	$ 1,218,481	$ 1,221,311
Operating income	22,898	24,602	29,718	17,519	17,187	22,644	21,721	17,584	94,737	79,136	89,975
Net income	$ 47,759	$ 14,407	$ 17,603	$ 11,387	$ 10,348	$ 13,826	$ 13,275	$ 10,575	$ 91,156	$ 48,024	$ 55,016
Basic earnings per share	$ 1.87	$ 0.56	$ 0.69	$ 0.45	$ 0.41	$ 0.55	$ 0.53	$ 0.42	$ 3.57	$ 1.92	$ 2.21
Diluted earnings per share	$ 1.82	$ 0.55	$ 0.68	$ 0.44	$ 0.40	$ 0.54	$ 0.52	$ 0.42	$ 3.49	$ 1.87	$ 2.16