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	Jun. 30, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 558,588	$ 585,404
Held-to-Maturity, Allowance for Credit Losses	(157)	(274)
Held-to-Maturity, Carrying Amount	558,431	585,130
Held-to-Maturity, Gross Unrecognized Gains	31,349	36,524
Held-to-Maturity, Gross Unrecognized Losses	(179)
Held-to-Maturity, Fair Value	589,601	621,654
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	483,630	494,374
Held-to-Maturity, Allowance for Credit Losses	(41)	(42)
Held-to-Maturity, Carrying Amount	483,589	494,332
Held-to-Maturity, Gross Unrecognized Gains	28,512	32,489
Held-to-Maturity, Gross Unrecognized Losses	(172)
Held-to-Maturity, Fair Value	511,929	526,821
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	54,344	69,981
Held-to-Maturity, Allowance for Credit Losses	(111)	(225)
Held-to-Maturity, Carrying Amount	54,233	69,756
Held-to-Maturity, Gross Unrecognized Gains	2,144	3,144
Held-to-Maturity, Fair Value	56,377	72,900
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	5,646	7,261
Held-to-Maturity, Carrying Amount	5,646	7,261
Held-to-Maturity, Gross Unrecognized Gains	506	645
Held-to-Maturity, Fair Value	6,152	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	14,838	13,626
Held-to-Maturity, Carrying Amount	14,838	13,626
Held-to-Maturity, Gross Unrecognized Gains	181	239
Held-to-Maturity, Gross Unrecognized Losses	(7)
Held-to-Maturity, Fair Value	15,012	13,865
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	130	162
Held-to-Maturity, Allowance for Credit Losses	(5)	(7)
Held-to-Maturity, Carrying Amount	125	155
Held-to-Maturity, Gross Unrecognized Gains	6	7
Held-to-Maturity, Fair Value	$ 131	$ 162