Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Totals, Amortized Cost	$ 977,543	$ 1,136,010
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	11,854	1,235
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	5,317	22,209
Debt Securities, Available-for-sale	984,080	1,115,036
Debt Securities Held to Maturity, Amortized Cost	393,449	444,936
Debt Securities Held to Maturity, Gross Unrealized Gains	6,732	409
Debt Securities Held to Maturity, Gross Unrealized Losses	(482)	(7,975)
Debt Securities Held to Maturity, Estimated Fair Value	399,699	437,370
Obligations of U.S. Government Agencies
Debt Securities, Available-for-sale [Line Items]
Totals, Amortized Cost	488,080	647,288
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8,004	771
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	447	18,078
Debt Securities, Available-for-sale	495,637	629,981
Debt Securities Held to Maturity, Amortized Cost	379,634	430,343
Debt Securities Held to Maturity, Gross Unrealized Gains	6,382	327
Debt Securities Held to Maturity, Gross Unrealized Losses	(482)	(7,745)
Debt Securities Held to Maturity, Estimated Fair Value	385,534	422,925
Obligations of States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Totals, Amortized Cost	108,360	128,890
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3,550	294
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	3,112
Debt Securities, Available-for-sale	111,910	126,072
Debt Securities Held to Maturity, Amortized Cost	13,815	14,593
Debt Securities Held to Maturity, Gross Unrealized Gains	350	82
Debt Securities Held to Maturity, Gross Unrealized Losses	0	(230)
Debt Securities Held to Maturity, Estimated Fair Value	14,165	14,445
Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Totals, Amortized Cost	4,420	4,381
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	108	97
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0
Debt Securities, Available-for-sale	4,528	4,478
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Totals, Amortized Cost	376,683	355,451
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	192	73
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,870	1,019
Debt Securities, Available-for-sale	$ 372,005	$ 354,505

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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Amount, before tax, of unrealized loss 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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Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

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