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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, 2020	Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 572,796	$ 599,421
Held-to-Maturity, Allowance for Credit Losses	(288)	0
Held-to-Maturity, Carrying Amount	572,508	599,421
Held-to-Maturity, Gross Unrecognized Gains	34,839	22,128
Held-to-Maturity, Gross Unrecognized Losses	(8)	(206)
Held-to-Maturity, Fair Value	607,339	621,343
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	471,913	466,270
Held-to-Maturity, Allowance for Credit Losses	(38)
Held-to-Maturity, Carrying Amount	471,875
Held-to-Maturity, Gross Unrecognized Gains	30,756	19,570
Held-to-Maturity, Gross Unrecognized Losses	(8)	(193)
Held-to-Maturity, Fair Value	502,623	485,647
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	70,721	109,241
Held-to-Maturity, Allowance for Credit Losses	(242)
Held-to-Maturity, Carrying Amount	70,479
Held-to-Maturity, Gross Unrecognized Gains	3,106	1,684
Held-to-Maturity, Fair Value	73,585	110,925
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	8,070	10,967
Held-to-Maturity, Carrying Amount	8,070
Held-to-Maturity, Gross Unrecognized Gains	644	544
Held-to-Maturity, Fair Value	8,714	11,511
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	21,914	12,723
Held-to-Maturity, Carrying Amount	21,914
Held-to-Maturity, Gross Unrecognized Gains	326	330
Held-to-Maturity, Gross Unrecognized Losses		(12)
Held-to-Maturity, Fair Value	22,240	13,041
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	178	220
Held-to-Maturity, Allowance for Credit Losses	(8)
Held-to-Maturity, Carrying Amount	170
Held-to-Maturity, Gross Unrecognized Gains	7
Held-to-Maturity, Gross Unrecognized Losses		(1)
Held-to-Maturity, Fair Value	$ 177	$ 219

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