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, 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
Earnings Per Common Share:
Net income available to common shareholders	$ 73,930	$ 66,387	$ 69,630	$ 38,793	$ 22,926	$ 36,966	$ 48,268	$ 44,298	$ 248,740	$ 152,458	$ 160,958
Basic weighted-average number of common shares outstanding (in shares)	331,492,648	331,486,500	331,318,381	330,727,416	264,332,895	264,058,174	263,958,292	263,797,024	331,258,964	264,038,123	254,841,571
Plus: Common stock equivalents (in shares)									1,434,754	850,884	426,765
Diluted weighted-average number of common shares outstanding (in shares)	332,856,385	333,000,242	332,895,483	332,465,527	265,288,067	264,936,220	264,778,242	264,546,266	332,693,718	264,889,007	255,268,336
Earnings per common share:
Basic (in usd per share)	$ 0.22	$ 0.20	$ 0.21	$ 0.12	$ 0.09	$ 0.14	$ 0.18	$ 0.17	$ 0.75	$ 0.58	$ 0.63
Diluted (in usd per share)	$ 0.22	$ 0.20	$ 0.21	$ 0.12	$ 0.09	$ 0.14	$ 0.18	$ 0.17	$ 0.75	$ 0.58	$ 0.63