Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 585,404	$ 599,421
Held-to-Maturity, Allowance for Credit Losses	(274)	0
Held-to-Maturity, Carrying Amount	585,130	599,421
Held-to-Maturity, Gross Unrecognized Gains	36,524	22,128
Held-to-Maturity, Gross Unrecognized Losses		(206)
Held-to-Maturity, Fair Value	621,654	621,343
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	494,374	466,270
Held-to-Maturity, Allowance for Credit Losses	(42)
Held-to-Maturity, Carrying Amount	494,332
Held-to-Maturity, Gross Unrecognized Gains	32,489	19,570
Held-to-Maturity, Gross Unrecognized Losses		(193)
Held-to-Maturity, Fair Value	526,821	485,647
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	69,981	109,241
Held-to-Maturity, Allowance for Credit Losses	(225)
Held-to-Maturity, Carrying Amount	69,756
Held-to-Maturity, Gross Unrecognized Gains	3,144	1,684
Held-to-Maturity, Fair Value	72,900	110,925
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	7,261	10,967
Held-to-Maturity, Carrying Amount	7,261
Held-to-Maturity, Gross Unrecognized Gains	645	544
Held-to-Maturity, Fair Value	7,906	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	13,626	12,723
Held-to-Maturity, Carrying Amount	13,626
Held-to-Maturity, Gross Unrecognized Gains	239	330
Held-to-Maturity, Gross Unrecognized Losses		(12)
Held-to-Maturity, Fair Value	13,865	13,041
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	162	220
Held-to-Maturity, Allowance for Credit Losses	(7)
Held-to-Maturity, Carrying Amount	155
Held-to-Maturity, Gross Unrecognized Gains	7
Held-to-Maturity, Gross Unrecognized Losses		(1)
Held-to-Maturity, Fair Value	$ 162	$ 219