Securities - Contractual Maturity Information for Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Due in 1 year or less, Amortized Cost	$ 14,151	$ 15,323
Due after 1 year through 5 years, Amortized Cost	93,532	100,623
Due after 5 years through 10 years, Amortized Cost	414,223	394,591
Due after 10 years, Amortized Cost	592,254	605,565
Total state and political subdivisions, Amortized Cost	1,114,160	1,116,102
Held to maturity securities, allowance for credit losses	3,235	0
Total state and political subdivisions, net of allowance for credit losses	1,110,925	1,116,102
Due in 1 year or less, Gross Unrealized Gains	20	5
Due after 1 year through 5 years, Gross Unrealized Gains	2,785	374
Due after 5 years through 10 years, Gross Unrealized Gains	11,534	389
Due after 10 years, Gross Unrealized Gains	22,230	494
Total state and political subdivisions, Gross Unrealized Gains	36,569	1,262
Due in 1 year or less, Gross Unrealized Losses		(60)
Due after 1 year through 5 years, Gross Unrealized Losses	(135)	(699)
Due after 5 years through 10 years, Gross Unrealized Losses	(760)	(8,400)
Due after 10 years, Gross Unrealized Losses	(2,925)	(25,860)
Total state and political subdivisions, Gross Unrealized Losses	(3,820)	(35,019)
Due in 1 year or less, Fair Value	14,171	15,268
Due after 1 year through 5 years, Fair Value	96,182	100,298
Due after 5 years through 10 years, Fair Value	424,997	386,580
Due after 10 years, Fair Value	611,559	580,199
Total state and political subdivisions, Fair Value	$ 1,146,909	$ 1,082,345

Held To Maturity Securities Debt Maturities After Five Through Ten Years Unrealized Gains

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Held To Maturity Securities Debt Maturities Within One Year Unrealized Gains

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Held-to-maturity securities, net of allowance for credit losses.

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Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in next fiscal year through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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