Investments - Gross Unrecognized Gains and Losses and Cost or Amortized Cost, Allowance for Credit Losses, Carrying Amount and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 541,396		$ 585,404
Held-to-Maturity, Allowance for Credit Losses	(172)	$ (157)	(274)
Held-to-Maturity, Carrying Amount	541,224		585,130
Held-to-Maturity, Gross Unrecognized Gains	28,387		36,524
Held-to-Maturity, Gross Unrecognized Losses	(166)
Held-to-Maturity, Fair Value	569,445		621,654
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	469,445		494,374
Held-to-Maturity, Allowance for Credit Losses	(45)	(41)	(42)
Held-to-Maturity, Carrying Amount	469,400		494,332
Held-to-Maturity, Gross Unrecognized Gains	25,849		32,489
Held-to-Maturity, Gross Unrecognized Losses	(158)
Held-to-Maturity, Fair Value	495,091		526,821
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	51,974		69,981
Held-to-Maturity, Allowance for Credit Losses	(123)	(111)	(225)
Held-to-Maturity, Carrying Amount	51,851		69,756
Held-to-Maturity, Gross Unrecognized Gains	1,892		3,144
Held-to-Maturity, Gross Unrecognized Losses	(4)
Held-to-Maturity, Fair Value	53,739		72,900
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	5,060		7,261
Held-to-Maturity, Carrying Amount	5,060		7,261
Held-to-Maturity, Gross Unrecognized Gains	474		645
Held-to-Maturity, Fair Value	5,534		7,906
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	14,801		13,626
Held-to-Maturity, Carrying Amount	14,801		13,626
Held-to-Maturity, Gross Unrecognized Gains	167		239
Held-to-Maturity, Gross Unrecognized Losses	(4)
Held-to-Maturity, Fair Value	14,964		13,865
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	116		162
Held-to-Maturity, Allowance for Credit Losses	(4)	$ (5)	(7)
Held-to-Maturity, Carrying Amount	112		155
Held-to-Maturity, Gross Unrecognized Gains	5		7
Held-to-Maturity, Fair Value	$ 117		$ 162