Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities (Detail) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 643,847,647	$ 586,609,439
Held to Maturity, Gross Unrealized Gains	38,978,000	46,604,000
Held to Maturity, Gross Unrealized Losses	1,704,000	572,000
Total held to maturity, Estimated Fair Value	681,122,000	632,641,000
Available for Sale, Amortized Cost	506,334,000	534,958,000
Available for Sale, Gross Unrealized Gains	17,335,000	20,512,000
Available for Sale, Gross Unrealized Losses	1,089,000	334,000
Available for Sale, Estimated Fair Value	522,580,000	555,136,000
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	89,716,000	77,435,000
Held to Maturity, Gross Unrealized Gains	2,839,000	3,984,000
Held to Maturity, Gross Unrealized Losses	649,000	223,000
Total held to maturity, Estimated Fair Value	91,906,000	81,196,000
Available for Sale, Amortized Cost	22,508,000	47,512,000
Available for Sale, Gross Unrealized Gains	257,000	424,000
Available for Sale, Gross Unrealized Losses	439,000	121,000
Available for Sale, Estimated Fair Value	22,326,000	47,815,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	346,517,000	312,319,000
Held to Maturity, Gross Unrealized Gains	20,073,000	23,212,000
Held to Maturity, Gross Unrealized Losses	788,000	143,000
Total held to maturity, Estimated Fair Value	365,802,000	335,388,000
Available for Sale, Amortized Cost	59,378,000	66,287,000
Available for Sale, Gross Unrealized Gains	2,366,000	2,690,000
Available for Sale, Gross Unrealized Losses	31,000	12,000
Available for Sale, Estimated Fair Value	61,713,000	68,965,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	188,162,000	173,270,000
Held to Maturity, Gross Unrealized Gains	15,061,000	18,172,000
Held to Maturity, Gross Unrealized Losses	267,000	206,000
Total held to maturity, Estimated Fair Value	202,956,000	191,236,000
Available for Sale, Amortized Cost	201,634,000	202,396,000
Available for Sale, Gross Unrealized Gains	9,644,000	10,496,000
Available for Sale, Gross Unrealized Losses	202,000	184,000
Available for Sale, Estimated Fair Value	211,076,000	212,708,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	19,453,000	23,585,000
Held to Maturity, Gross Unrealized Gains	1,005,000	1,236,000
Held to Maturity, Gross Unrealized Losses	0	0
Total held to maturity, Estimated Fair Value	20,458,000	24,821,000
Available for Sale, Amortized Cost	222,814,000	218,763,000
Available for Sale, Gross Unrealized Gains	5,068,000	6,902,000
Available for Sale, Gross Unrealized Losses	417,000	17,000
Available for Sale, Estimated Fair Value	$ 227,465,000	$ 225,648,000