Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	$ 125,901	$ 101,522
Available-for-sale, Gross Unrealized Gains	1,526	163
Available-for-sale, Gross Unrealized Losses	(119)	(968)
Available-for-sale, Estimated Fair Value	127,308	100,717
Held-to-maturity, Amortized Cost	823,717	851,903
Held-to-maturity, Gross Unrealized Gains	8,192	1,303
Held-to-maturity, Gross Unrealized Losses	(4,945)	(20,391)
Estimated Fair Value	826,964	832,815
Total, Amortized Cost	949,618	953,425
Total, Gross Unrealized Gains	9,718	1,466
Total, Gross Unrealized Losses	(5,064)	(21,359)
Total, Estimated Fair Value	954,272	933,532
Investment securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	125,300
Available-for-sale, Gross Unrealized Gains	1,521
Available-for-sale, Gross Unrealized Losses	(119)
Available-for-sale, Estimated Fair Value	126,702
Held-to-maturity, Amortized Cost	225,840	205,796
Held-to-maturity, Gross Unrealized Gains	2,164	193
Held-to-maturity, Gross Unrealized Losses	(3,630)	(6,811)
Estimated Fair Value	224,374	199,178
U.S. government and agency obligations \| Investment securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	124,050	100,524
Available-for-sale, Gross Unrealized Gains	1,520	163
Available-for-sale, Gross Unrealized Losses	(119)	(963)
Available-for-sale, Estimated Fair Value	125,451	99,724
Held-to-maturity, Amortized Cost	9,981	14,975
Held-to-maturity, Gross Unrealized Gains	2	0
Held-to-maturity, Gross Unrealized Losses	(2)	(130)
Estimated Fair Value	9,981	14,845
State and municipal obligations \| Investment securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	1,250
Available-for-sale, Gross Unrealized Gains	1
Available-for-sale, Gross Unrealized Losses	0
Available-for-sale, Estimated Fair Value	1,251
Held-to-maturity, Amortized Cost	134,626	123,987
Held-to-maturity, Gross Unrealized Gains	1,247	67
Held-to-maturity, Gross Unrealized Losses	(259)	(1,697)
Estimated Fair Value	135,614	122,357
Corporate debt securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	46,029
Estimated Fair Value	42,660
Corporate debt securities \| Investment securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	81,233	66,834
Held-to-maturity, Gross Unrealized Gains	915	126
Held-to-maturity, Gross Unrealized Losses	(3,369)	(4,984)
Estimated Fair Value	78,779	61,976
Mortgage-backed securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	597,877	646,107
Held-to-maturity, Gross Unrealized Gains	6,028	1,110
Held-to-maturity, Gross Unrealized Losses	(1,315)	(13,580)
Estimated Fair Value	602,590	633,637
Mortgage-backed securities \| FHLMC
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	219,047	237,703
Held-to-maturity, Gross Unrealized Gains	2,194	159
Held-to-maturity, Gross Unrealized Losses	(569)	(5,110)
Estimated Fair Value	220,672	232,752
Mortgage-backed securities \| FNMA
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	601	998
Available-for-sale, Gross Unrealized Gains	5	0
Available-for-sale, Gross Unrealized Losses	0	(5)
Available-for-sale, Estimated Fair Value	606	993
Held-to-maturity, Amortized Cost	259,192	277,266
Held-to-maturity, Gross Unrealized Gains	2,566	753
Held-to-maturity, Gross Unrealized Losses	(503)	(6,030)
Estimated Fair Value	261,255	271,989
Mortgage-backed securities \| GNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	116,730	127,611
Held-to-maturity, Gross Unrealized Gains	1,268	198
Held-to-maturity, Gross Unrealized Losses	(169)	(2,360)
Estimated Fair Value	117,829	125,449
Mortgage-backed securities \| SBA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	2,908	3,527
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(74)	(80)
Estimated Fair Value	$ 2,834	$ 3,447

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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