Investment Securities Held-to-Maturity - Summary of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,110,555	$ 2,046,647
Gross Unrealized Gains	5,759	2,660
Gross Unrealized Losses	41,871	57,886
Estimated Fair Value	2,074,443	1,991,421
U.S. Treasury [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		9,960
Gross Unrealized Losses		2
Estimated Fair Value		9,958
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	201,327	234,228
Gross Unrealized Gains	453	336
Gross Unrealized Losses	417	803
Estimated Fair Value	201,363	233,761
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	50,797	52,051
Gross Unrealized Losses	1,295	2,065
Estimated Fair Value	49,502	49,986
Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,858,431	1,750,408
Gross Unrealized Gains	5,306	2,324
Gross Unrealized Losses	40,159	55,016
Estimated Fair Value	$ 1,823,578	$ 1,697,716