Summary of Significant Accounting Policies - Calculation of Both Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, $ 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
Earnings Per Common Share:
Net income available to common shareholders	$ 48,293	$ 41,045	$ 37,230	$ 34,390	$ 2,875	$ 33,937	$ 31,991	$ 30,341	$ 160,958	$ 99,144	$ 116,172
Basic weighted-average number of common shares outstanding (in shares)	256,422,437	254,473,994	254,381,170	254,075,349	239,916,562	232,737,953	232,565,404	232,338,775	254,841,571	234,405,909	205,716,293
Plus: Common stock equivalents (in shares)									426,765	31,091	0
Diluted weighted-average number of common shares outstanding (in shares)	256,952,036	254,940,307	254,771,213	254,347,420	239,972,546	232,780,219	232,586,616	232,341,921	255,268,336	234,437,000	205,716,293
Earnings per common share:
Basic (in usd per share)	$ 0.19	$ 0.16	$ 0.15	$ 0.14	$ 0.01	$ 0.15	$ 0.14	$ 0.13	$ 0.63	$ 0.42	$ 0.56
Diluted (in usd per share)	$ 0.19	$ 0.16	$ 0.15	$ 0.14	$ 0.01	$ 0.15	$ 0.14	$ 0.13	$ 0.63	$ 0.42	$ 0.56