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, 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
Income Statement [Abstract]
Operating revenue	$ 300,223	$ 316,442	$ 311,905	$ 289,911	$ 287,610	$ 317,199	$ 323,480	$ 293,022	$ 1,218,481	$ 1,221,311	$ 1,272,321
Operating income	17,187	22,644	21,721	17,584	17,616	19,845	31,281	21,233	79,136	89,975	85,693
Net income	$ 10,348	$ 13,826	$ 13,275	$ 10,575	$ 11,387	$ 11,777	$ 19,247	$ 12,605	$ 48,024	$ 55,016	$ 51,991
Basic earnings per share	$ 0.41	$ 0.55	$ 0.53	$ 0.42	$ 0.46	$ 0.47	$ 0.77	$ 0.51	$ 1.92	$ 2.21	$ 2.12
Diluted earnings per share	$ 0.40	$ 0.54	$ 0.52	$ 0.42	$ 0.45	$ 0.46	$ 0.75	$ 0.49	$ 1.87	$ 2.16	$ 2.04