Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 229,986	$ 241,625	$ 221,312	$ 232,389	$ 214,042	$ 226,165	$ 210,081	$ 220,147	$ 925,312	$ 870,435	$ 821,654
Income from operations	82,392	84,824	75,865	93,636	77,199	77,628	72,090	88,257
Consolidated net income	47,974	54,865	44,463	63,075	42,381	46,757	40,804	57,190	210,377	187,132	150,512
Net income available for Common Stockholders	$ 44,993	$ 48,525	$ 39,498	$ 56,888	$ 36,966	$ 40,998	$ 35,490	$ 50,583	$ 210,377	$ 187,132	$ 150,512
Weighted average Common Shares outstanding – basic (shares)	88,115	87,037	86,763	86,048	85,163	85,105	84,516	84,321	86,997	84,778	84,031
Weighted average Common Shares outstanding—Diluted (shares)	94,295	93,324	93,063	93,011	92,965	92,910	92,264	92,041	93,425	92,569	91,907
Net income per Common Share outstanding—Basic (usd per share)	$ 0.51	$ 0.56	$ 0.46	$ 0.66	$ 0.43	$ 0.48	$ 0.42	$ 0.60	$ 2.18	$ 1.93	$ 1.55
Net income per Common Share outstanding—Diluted (usd per share)	$ 0.51	$ 0.56	$ 0.45	$ 0.65	$ 0.43	$ 0.48	$ 0.42	$ 0.60	$ 2.17	$ 1.92	$ 1.54