Securities - Contractual Maturity Information for Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Due in 1 year or less, Amortized Cost	$ 4,907	$ 15,323
Due after 1 year through 5 years, Amortized Cost	123,643	100,623
Due after 5 years through 10 years, Amortized Cost	423,759	394,591
Due after 10 years, Amortized Cost	462,305	605,565
Total state and political subdivisions, Amortized Cost	1,014,614	1,116,102
Held to maturity securities, allowance for credit losses	(2,610)	0
Total state and political subdivisions, net of allowance for credit losses	1,012,004	1,116,102
Due in 1 year or less, Gross Unrealized Gains	29	5
Due after 1 year through 5 years, Gross Unrealized Gains	3,402	374
Due after 5 years through 10 years, Gross Unrealized Gains	12,845	389
Due after 10 years, Gross Unrealized Gains	13,447	494
Total state and political subdivisions, Gross Unrealized Gains	29,723	1,262
Due in 1 year or less, Gross Unrealized Losses		(60)
Due after 1 year through 5 years, Gross Unrealized Losses	(144)	(699)
Due after 5 years through 10 years, Gross Unrealized Losses	(1,566)	(8,400)
Due after 10 years, Gross Unrealized Losses	(13,183)	(25,860)
Total state and political subdivisions, Gross Unrealized Losses	(14,893)	(35,019)
Due in 1 year or less, Fair Value	4,936	15,268
Due after 1 year through 5 years, Fair Value	126,901	100,298
Due after 5 years through 10 years, Fair Value	435,038	386,580
Due after 10 years, Fair Value	462,569	580,199
Total state and political subdivisions, Fair Value	$ 1,029,444	$ 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 current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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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 second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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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 after tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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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 following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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