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, 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
Income Statement [Abstract]
Operating revenue	$ 287,610	$ 317,199	$ 323,480	$ 293,022	$ 309,648	$ 332,544	$ 330,399	$ 299,730	$ 1,221,311	$ 1,272,321	$ 1,139,094
Operating income	17,616	19,845	31,281	21,233	20,572	27,149	22,741	15,231	89,975	85,693	74,418
Net income	$ 11,387	$ 11,777	$ 19,247	$ 12,605	$ 13,569	$ 16,278	$ 13,568	$ 8,576	$ 55,016	$ 51,991	$ 43,627
Basic earnings per share	$ 0.46	$ 0.47	$ 0.77	$ 0.51	$ 0.55	$ 0.66	$ 0.55	$ 0.35	$ 2.21	$ 2.12	$ 1.81
Diluted earnings per share	$ 0.46	$ 0.46	$ 0.75	$ 0.49	$ 0.53	$ 0.64	$ 0.53	$ 0.34	$ 2.16	$ 2.04	$ 1.73