Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	$ 42,964	$ 4,648
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	852	5
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	42,964	4,648
Available-for Sale, Gross Unrealized Losses, Total	852	5
States and Political Subdivisions [Member]
Marketable Securities [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	21,199
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	402
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	21,199
Available-for Sale, Gross Unrealized Losses, Total	402
Corporate Bonds [Member]
Marketable Securities [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	8,678	2,515
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	180	3
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	8,678	2,515
Available-for Sale, Gross Unrealized Losses, Total	180	3
U.S. Agency-Based Mortgage-Backed Securities [Member]
Marketable Securities [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	1,595	2,133
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	8	2
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	1,595	2,133
Available-for Sale, Gross Unrealized Losses, Total	8	$ 2
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Marketable Securities [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	11,492
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	262
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	11,492
Available-for Sale, Gross Unrealized Losses, Total	$ 262

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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-Publisher FASB
-Name Accounting Standards Codification
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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

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