Investments - Gross Unrecognized Gains and Losses and Cost or Amortized Cost, Allowance for Credit Losses and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 608,590	$ 599,421
Held-to-Maturity, Gross Unrecognized Gains	24,308	22,128
Held-to-Maturity, Gross Unrecognized Losses	(935)	(206)
Held-to-Maturity, Allowance for Credit Losses	(275)	0
Held-to-Maturity, Fair Value	631,688	621,343
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	483,340	466,270
Held-to-Maturity, Gross Unrecognized Gains	21,875	19,570
Held-to-Maturity, Gross Unrecognized Losses	(145)	(193)
Held-to-Maturity, Allowance for Credit Losses	(50)
Held-to-Maturity, Fair Value	505,020	485,647
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	100,969	109,241
Held-to-Maturity, Gross Unrecognized Gains	1,200	1,684
Held-to-Maturity, Gross Unrecognized Losses	(790)
Held-to-Maturity, Allowance for Credit Losses	(215)
Held-to-Maturity, Fair Value	101,164	110,925
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	10,352	10,967
Held-to-Maturity, Gross Unrecognized Gains	720	544
Held-to-Maturity, Fair Value	11,072	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	13,721	12,723
Held-to-Maturity, Gross Unrecognized Gains	513	330
Held-to-Maturity, Gross Unrecognized Losses		(12)
Held-to-Maturity, Fair Value	14,234	13,041
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	208	220
Held-to-Maturity, Gross Unrecognized Losses		(1)
Held-to-Maturity, Allowance for Credit Losses	(10)
Held-to-Maturity, Fair Value	$ 198	$ 219