Investment Securities Held-to-Maturity - Summary of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,178,568	$ 2,046,647
Gross Unrealized Gains	22,004	2,660
Gross Unrealized Losses	16,047	57,886
Estimated Fair Value	2,184,525	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	162,870	234,228
Gross Unrealized Gains	770	336
Gross Unrealized Losses	72	803
Estimated Fair Value	163,568	233,761
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	48,666	52,051
Gross Unrealized Gains	288
Gross Unrealized Losses	224	2,065
Estimated Fair Value	48,730	49,986
Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,967,032	1,750,408
Gross Unrealized Gains	20,946	2,324
Gross Unrealized Losses	15,751	55,016
Estimated Fair Value	$ 1,972,227	$ 1,697,716