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	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 575,274	$ 585,404
Held-to-Maturity, Allowance for Credit Losses	(166)	(274)
Held-to-Maturity, Carrying Amount	575,108	585,130
Held-to-Maturity, Gross Unrecognized Gains	29,388	36,524
Held-to-Maturity, Gross Unrecognized Losses	(776)
Held-to-Maturity, Fair Value	603,720	621,654
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	486,376	494,374
Held-to-Maturity, Allowance for Credit Losses	(44)	(42)
Held-to-Maturity, Carrying Amount	486,332	494,332
Held-to-Maturity, Gross Unrecognized Gains	26,179	32,489
Held-to-Maturity, Gross Unrecognized Losses	(774)
Held-to-Maturity, Fair Value	511,737	526,821
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	68,518	69,981
Held-to-Maturity, Allowance for Credit Losses	(116)	(225)
Held-to-Maturity, Carrying Amount	68,402	69,756
Held-to-Maturity, Gross Unrecognized Gains	2,440	3,144
Held-to-Maturity, Fair Value	70,842	72,900
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	6,357	7,261
Held-to-Maturity, Carrying Amount	6,357	7,261
Held-to-Maturity, Gross Unrecognized Gains	568	645
Held-to-Maturity, Fair Value	6,925	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	13,877	13,626
Held-to-Maturity, Carrying Amount	13,877	13,626
Held-to-Maturity, Gross Unrecognized Gains	195	239
Held-to-Maturity, Gross Unrecognized Losses	(2)
Held-to-Maturity, Fair Value	14,070	13,865
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	146	162
Held-to-Maturity, Allowance for Credit Losses	(6)	(7)
Held-to-Maturity, Carrying Amount	140	155
Held-to-Maturity, Gross Unrecognized Gains	6	7
Held-to-Maturity, Fair Value	$ 146	$ 162