Quarterly Financial Data - Schedule of Quarterly Financial Information (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
Disclosure Quarterly Financial Data Schedule Of Quarterly Financial Information [Abstract]
Revenues	$ 104,567	$ 91,913	$ 92,682	$ 170,081	$ 120,380	$ 91,619	$ 100,497	$ 186,337	$ 459,244	$ 498,834	$ 444,306
Operating Income	16,171	10,909	13,170	37,508	12,408	7,792	10,457	31,623	77,758	62,279	62,734
Net Income	$ 8,619	$ 5,119	$ 6,294	$ 21,109	$ 10,097	$ 3,180	$ 5,134	$ 17,681	$ 41,140	$ 36,092	$ 32,787
Earnings per share:
Basic (in usd per share)	$ 0.56	$ 0.34	$ 0.41	$ 1.45	$ 0.69	$ 0.22	$ 0.35	$ 1.22	$ 2.73	$ 2.48	$ 2.27
Diluted (in usd per share)	$ 0.56	$ 0.33	$ 0.41	$ 1.44	$ 0.69	$ 0.22	$ 0.35	$ 1.21	$ 2.72	$ 2.47	$ 2.26