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, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Disclosure Quarterly Financial Data Schedule Of Quarterly Financial Information [Abstract]
Revenues	$ 122,887	[1]	$ 86,545	[1]	$ 94,146	[1]	$ 140,729	[1]	$ 109,516	[1]	$ 78,175	[1]	$ 83,897	[1]	$ 120,914	[1]	$ 444,306	$ 392,502	$ 418,027
Operating Income	18,312	[1]	8,720	[1]	9,152	[1]	26,550	[1]	18,543	[1]	7,564	[1]	10,455	[1]	20,073	[1]	62,734	56,635	53,705
Net Income	$ 9,683	[1]	$ 3,879	[1]	$ 4,356	[1]	$ 14,869	[1]	$ 9,857	[1]	$ 3,219	[1]	$ 5,060	[1]	$ 10,727	[1]	$ 32,787	$ 28,863	$ 27,622
Earnings per share:
Basic (in usd per share)	$ 1.01	[1]	$ 0.40	[1]	$ 0.45	[1]	$ 1.55	[1]	$ 1.03	[1]	$ 0.34	[1]	$ 0.53	[1]	$ 1.12	[1]	$ 3.41	$ 3.01	$ 2.89
Diluted (in usd per share)	$ 1.00	[1]	$ 0.40	[1]	$ 0.45	[1]	$ 1.54	[1]	$ 1.02	[1]	$ 0.33	[1]	$ 0.52	[1]	$ 1.11	[1]	$ 3.39	$ 2.99	$ 2.87

[1] The sum of the four quarters does not equal the total year due to rounding.