Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 214,042	$ 226,165	$ 210,081	$ 220,147	$ 201,616	$ 210,144	$ 201,480	$ 208,414	$ 870,435	$ 821,654	$ 776,809
Income from operations	77,199	77,628	72,090	88,257	72,617	72,512	68,097	82,014
Consolidated net income	42,381	46,757	40,804	57,190	39,767	42,106	36,826	31,813	187,132	150,512	138,468
Net income available for Common Stockholders	$ 36,966	$ 40,998	$ 35,490	$ 50,583	$ 34,501	$ 36,673	$ 31,786	$ 27,185	$ 187,132	$ 150,512	$ 138,468
Weighted average Common Shares outstanding – basic (shares)	85,163	85,015	84,516	84,321	84,072	84,057	84,031	83,961	84,778	84,031	83,362
Weighted average Common Shares outstanding—Diluted (shares)	92,965	92,910	92,264	92,041	91,875	91,940	91,851	91,777	92,569	91,907	91,511
Net income per Common Share outstanding—Basic (usd per share)	$ 0.43	$ 0.48	$ 0.42	$ 0.60	$ 0.41	$ 0.44	$ 0.38	$ 0.32	$ 1.93	$ 1.55	$ 1.42
Net income per Common Share outstanding—Diluted (usd per share)	$ 0.43	$ 0.48	$ 0.42	$ 0.60	$ 0.41	$ 0.43	$ 0.38	$ 0.32	$ 1.92	$ 1.54	$ 1.41