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, 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 112,137	$ 117,841	$ 112,749	$ 107,871	$ 106,498	$ 100,409	$ 96,140	$ 86,168	$ 450,598	$ 389,215	$ 280,086
Operating Income	36,665	38,927	22,502	32,626	33,892	31,630	26,882	21,100	130,720	113,504	60,445
Net Income Attributable to Common Stockholders	$ 18,879	$ 37,340	$ 19,543	$ 21,938	$ 24,756	$ 21,089	$ 15,385	$ 13,960	$ 97,700	$ 75,190	$ 37,608
Earnings per share-Basic	$ 2.09	$ 4.10	$ 2.14	$ 2.41	$ 2.72	$ 2.64	$ 1.97	$ 1.79	$ 10.75	$ 9.18	$ 4.87
Earnings per share-Diluted	$ 2.05	$ 4.02	$ 2.10	$ 2.34	$ 2.65	$ 2.56	$ 1.91	$ 1.73	$ 10.51	$ 8.92	$ 4.66