Securities - Contractual Maturity Information for Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Due in 1 year or less, Amortized Cost	$ 1,372	$ 4,907
Due after 1 year through 5 years, Amortized Cost	121,217	123,643
Due after 5 years through 10 years, Amortized Cost	416,437	423,759
Due after 10 years, Amortized Cost	503,644	462,305
Total state and political subdivisions, Amortized Cost	1,042,670	1,014,614
Allowance for credit losses	(2,959)	(2,610)
Total state and political subdivisions, net of allowance for credit losses	1,039,711	1,012,004
Due in 1 year or less, Gross Unrealized Gains	15	29
Due after 1 year through 5 years, Gross Unrealized Gains	1,703	3,402
Due after 5 years through 10 years, Gross Unrealized Gains	4,020	12,845
Due after 10 years, Gross Unrealized Gains	1,821	13,447
Total state and political subdivisions, Gross Unrealized Gains	7,559	29,723
Due after 1 year through 5 years, Gross Unrealized Losses	(137)	(144)
Due after 5 years through 10 years, Gross Unrealized Losses	(6,858)	(1,566)
Due after 10 years, Gross Unrealized Losses	(31,105)	(13,183)
Total state and political subdivisions, Gross Unrealized Losses	(38,100)	(14,893)
Due in 1 year or less, Fair Value	1,387	4,936
Due after 1 year through 5 years, Fair Value	122,783	126,901
Due after 5 years through 10 years, Fair Value	413,599	435,038
Due after 10 years, Fair Value	474,360	462,569
Total state and political subdivisions, Fair Value	$ 1,012,129	$ 1,029,444

Held to maturity securities debt maturities after five through ten years unrealized gains.

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Held to maturity securities debt maturities after five through ten years unrealized losses.

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Held to maturity securities debt maturities after one through five years unrealized gains.

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Held to maturity securities debt maturities after one through five years unrealized losses.

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Held to maturity securities debt maturities after 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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