Selected Quarterly Data - Summary of Selected Quarterly Data (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 106,498	$ 100,409	$ 96,140	$ 86,168	$ 78,919	$ 71,951	$ 66,818	$ 62,398	$ 389,215	$ 280,086	$ 204,652
Operating Income	33,892	31,630	26,882	21,100	20,472	17,767	14,253	7,953	113,504	60,445	13,903
Net Income Attributable to Common Stockholders	$ 24,756	$ 21,089	$ 15,385	$ 13,960	$ 12,213	$ 11,642	$ 8,367	$ 5,386	$ 75,190	$ 37,608	$ 111,678
Earnings per share-Basic	$ 2.72	$ 2.64	$ 1.97	$ 1.79	$ 1.56	$ 1.48	$ 1.08	$ 0.72	$ 9.18	$ 4.87	$ 17.98
Earnings per share-Diluted	$ 2.65	$ 2.56	$ 1.91	$ 1.73	$ 1.50	$ 1.43	$ 1.04	$ 0.68	$ 8.92	$ 4.66	$ 16.34