Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 243,451	$ 256,675	$ 240,502	$ 246,025	$ 229,986	$ 241,625	$ 221,312	$ 232,389	$ 986,653	$ 925,312	$ 870,435
Consolidated net income	53,380	59,660	49,169	64,177	47,974	54,865	44,463	63,075	226,386	210,377	187,132
Net income available for Common Stockholders	$ 50,166	$ 56,070	$ 46,137	$ 60,222	$ 44,993	$ 48,525	$ 39,498	$ 56,888	$ 226,386	$ 210,377	$ 187,132
Weighted average Common Shares outstanding – Basic (shares)	89,570	89,200	88,549	88,524	88,115	87,037	86,763	86,048	88,964	86,997	84,778
Weighted average Common Shares outstanding—Diluted (shares)	95,577	95,263	94,623	94,577	94,295	93,324	93,063	93,011	95,055	93,425	92,569
Net income per Common Share outstanding—Basic (usd per share)	$ 0.56	$ 0.63	$ 0.52	$ 0.68	$ 0.51	$ 0.56	$ 0.46	$ 0.66	$ 2.39	$ 2.18	$ 1.93
Net income per Common Share outstanding—Diluted (usd per share)	$ 0.56	$ 0.63	$ 0.52	$ 0.68	$ 0.51	$ 0.56	$ 0.45	$ 0.65	$ 2.38	$ 2.17	$ 1.92