Investments - Gross Unrealized Gains and Losses and Cost or Amortized Cost, Allowance for Credit Losses and Fair Value of Investments Classified as Available-for-Sale (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	$ 401,671,000	$ 425,698,000
Available-for-sale, Gross Unrealized Gains	17,694,000	15,613,000
Available-for-sale, Gross Unrealized Losses	(329,000)	(165,000)
Available-for-sale, Fair Value	419,036,000	441,146,000
Available-for-sale, Allowance for Credit Losses	0	0
States and Political Subdivisions [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	225,122,000	225,895,000
Available-for-sale, Gross Unrealized Gains	12,798,000	11,906,000
Available-for-sale, Gross Unrealized Losses	(6,000)	(26,000)
Available-for-sale, Fair Value	237,914,000	237,775,000
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	111,249,000	130,453,000
Available-for-sale, Gross Unrealized Gains	2,887,000	3,326,000
Available-for-sale, Gross Unrealized Losses	(323,000)	(1,000)
Available-for-sale, Fair Value	113,813,000	133,778,000
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	33,400,000	29,499,000
Available-for-sale, Gross Unrealized Gains	813,000	64,000
Available-for-sale, Gross Unrealized Losses		(96,000)
Available-for-sale, Fair Value	34,213,000	29,467,000
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale, Amortized Cost	31,900,000	39,851,000
Available-for-sale, Gross Unrealized Gains	1,196,000	317,000
Available-for-sale, Gross Unrealized Losses		(42,000)
Available-for-sale, Fair Value	$ 33,096,000	$ 40,126,000