Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Marketable Securities [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	$ 108,725	$ 196,433
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	5,471	10,625
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	177,138	63,424
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	20,516	7,849
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	285,863	259,857
Available-for Sale, Gross Unrealized Losses, Total	25,987	18,474
States and Political Subdivisions [Member]
Marketable Securities [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	55,911	87,522
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	3,207	5,319
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	68,237	24,980
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	11,271	4,507
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	124,148	112,502
Available-for Sale, Gross Unrealized Losses, Total	14,478	9,826
Corporate Bonds [Member]
Marketable Securities [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	51,905	98,590
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	2,247	4,549
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	90,991	30,011
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	6,977	2,108
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	142,896	128,601
Available-for Sale, Gross Unrealized Losses, Total	9,224	6,657
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		4,732
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		444
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,664	714
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	712	94
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	4,664	5,446
Available-for Sale, Gross Unrealized Losses, Total	712	538
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	909	5,589
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	17	313
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	13,246	7,719
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,556	1,140
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	14,155	13,308
Available-for Sale, Gross Unrealized Losses, Total	$ 1,573	$ 1,453

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 more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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