Quarterly Financial Data - Summarized Quarterly Financial Data (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
Selected Quarterly Financial Information [Abstract]
Net interest income	$ 113,016	$ 92,820	$ 72,688	$ 70,521	$ 71,275	$ 71,739	$ 70,483	$ 65,433	$ 349,045	$ 278,930	$ 257,074
Recapture of provision for loan losses	3,000	500	(1,000)	(1,000)	(1,500)	(1,500)	(1,000)	(4,500)
Net earnings	$ 43,159	$ 38,558	$ 35,373	$ 34,913	$ 17,851	$ 29,683	$ 28,373	$ 28,504	$ 152,003	$ 104,411	$ 101,429
Basic earnings per common share	$ 0.31	$ 0.30	$ 0.32	$ 0.32	$ 0.16	$ 0.27	$ 0.26	$ 0.26	$ 1.25	$ 0.95	$ 0.94
Diluted earnings per common share	$ 0.31	$ 0.30	$ 0.32	$ 0.32	$ 0.16	$ 0.27	$ 0.26	$ 0.26	$ 1.24	$ 0.95	$ 0.94