Selected Quarterly Data - Selected Quarterly Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Data [Line Items]
Revenues	$ 78,919	$ 71,951	$ 66,818	$ 62,398	$ 56,172	$ 55,457	$ 48,902	$ 44,121	$ 280,086	$ 204,652	$ 144,556
Operating income (loss)	20,472	17,767	14,253	7,953	8,194	(2,193)	3,459	4,443	60,445	13,903	9,271
Net Income (Loss) Attributable to Common Stockholders	$ 12,213	$ 11,642	$ 8,367	$ 5,386	$ 109,555	$ (3,483)	$ 2,043	$ 2,979	$ 37,608	$ 111,678	$ 5,209
Earnings (Loss) per share - Basic	$ 1.56	$ 1.48	$ 1.08	$ 0.72	$ 17.60	$ (0.56)	$ 0.33	$ 0.48	$ 4.87	$ 17.98	$ 0.87
Earnings (Loss) per share - Diluted	$ 1.50	$ 1.43	$ 1.04	$ 0.68	$ 16.35	$ (0.56)	$ 0.30	$ 0.43	$ 4.66	$ 16.34	$ 0.81