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, 2019	Dec. 31, 2018
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	$ 599,421	$ 613,878
Held-to-Maturity, Gross Unrealized Gains	22,128	5,860
Held-to-Maturity, Gross Unrealized Losses	(206)	(2,966)
Held-to-Maturity, Fair Value	621,343	616,772
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	466,270	445,922
Held-to-Maturity, Gross Unrealized Gains	19,570	5,109
Held-to-Maturity, Gross Unrealized Losses	(193)	(2,084)
Held-to-Maturity, Fair Value	485,647	448,947
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	109,241	91,762
Held-to-Maturity, Gross Unrealized Gains	1,684	62
Held-to-Maturity, Gross Unrealized Losses		(455)
Held-to-Maturity, Fair Value	110,925	91,369
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	10,967	8,102
Held-to-Maturity, Gross Unrealized Gains	544	327
Held-to-Maturity, Gross Unrealized Losses		(80)
Held-to-Maturity, Fair Value	11,511	8,349
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	12,723	67,042
Held-to-Maturity, Gross Unrealized Gains	330	340
Held-to-Maturity, Gross Unrealized Losses	(12)	(339)
Held-to-Maturity, Fair Value	13,041	67,043
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-maturity, Amortized Cost, Totals	220	1,050
Held-to-Maturity, Gross Unrealized Gains		22
Held-to-Maturity, Gross Unrealized Losses	(1)	(8)
Held-to-Maturity, Fair Value	$ 219	$ 1,064