Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Consolidated net income	$ 39,767	$ 42,106	$ 36,826	$ 31,813	$ 34,262	$ 30,276	$ 30,040	$ 43,890	$ 150,512	$ 138,468	$ 77,247
Total revenues	201,616	210,144	201,480	208,414	190,261	200,778	189,025	196,745	821,654	776,809	729,048
Income from operations	72,617	72,512	68,097	82,014	68,153	67,545	62,770	73,730
Net income available for Common Stockholders	$ 34,501	$ 36,673	$ 31,786	$ 27,185	$ 29,403	$ 25,746	$ 25,483	$ 38,099	$ 150,512	$ 138,468	$ 125,905
Weighted average Common Stockholders outstanding—basic	84,072	84,057	84,031	83,961	83,562	83,531	83,234	83,116	84,031	83,362	83,018
Weighted average Common Shares outstanding—Diluted	91,875	91,940	91,851	91,777	91,644	91,528	91,420	91,353	91,907	91,511	91,196
Net income per Common Share outstanding—Basic	$ 0.41	$ 0.44	$ 0.38	$ 0.32	$ 0.35	$ 0.31	$ 0.31	$ 0.46	$ 1.55	$ 1.42	$ 1.29
Net income per Common Share outstanding—Diluted	$ 0.41	$ 0.43	$ 0.38	$ 0.32	$ 0.35	$ 0.31	$ 0.30	$ 0.46	$ 1.54	$ 1.41	$ 1.28