Quarterly Financial Data - Schedule of Quarterly Financial Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure Quarterly Financial Data Schedule Of Quarterly Financial Information [Abstract]
Revenues	$ 120,380	$ 91,619	$ 100,497	$ 186,337	$ 122,887	$ 86,545	$ 94,146	$ 140,729	$ 498,834	$ 444,306	$ 392,502
Operating Income	12,408	7,792	10,457	31,623	18,312	8,720	9,152	26,550	62,279	62,734	56,635
Net Income	$ 10,097	$ 3,180	$ 5,134	$ 17,681	$ 9,683	$ 3,879	$ 4,356	$ 14,869	$ 36,092	$ 32,787	$ 28,863
Earnings per share:
Basic (in usd per share)	$ 0.69	$ 0.22	$ 0.35	$ 1.22	$ 0.67	$ 0.27	$ 0.30	$ 1.03	$ 2.48	$ 2.27	$ 2.01
Diluted (in usd per share)	$ 0.69	$ 0.22	$ 0.35	$ 1.21	$ 0.67	$ 0.27	$ 0.30	$ 1.03	$ 2.47	$ 2.26	$ 1.99