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, 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
Earnings Per Common Share:
Net income available to common shareholders	$ 22,926	$ 36,966	$ 48,268	$ 44,298	$ 48,293	$ 41,045	$ 37,230	$ 34,390	$ 152,458	$ 160,958	$ 99,144
Basic weighted-average number of common shares outstanding (in shares)	264,332,895	264,058,174	263,958,292	263,797,024	256,422,437	254,473,994	254,381,170	254,075,349	264,038,123	254,841,571	234,405,909
Plus: Common stock equivalents (in shares)									850,884	426,765	31,091
Diluted weighted-average number of common shares outstanding (in shares)	265,288,067	264,936,220	264,778,242	264,546,266	256,952,036	254,940,307	254,771,213	254,347,420	264,889,007	255,268,336	234,437,000
Earnings per common share:
Basic (in usd per share)	$ 0.09	$ 0.14	$ 0.18	$ 0.17	$ 0.19	$ 0.16	$ 0.15	$ 0.14	$ 0.58	$ 0.63	$ 0.42
Diluted (in usd per share)	$ 0.09	$ 0.14	$ 0.18	$ 0.17	$ 0.19	$ 0.16	$ 0.15	$ 0.14	$ 0.58	$ 0.63	$ 0.42