Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities (Detail) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 661,080,363	$ 586,609,439
Held to Maturity, Gross Unrealized Gains	33,949,000	46,604,000
Held to Maturity, Gross Unrealized Losses	2,922,000	572,000
Total held to maturity, Estimated Fair Value	692,107,000	632,641,000
Available for Sale, Amortized Cost	506,513,000	534,958,000
Available for Sale, Gross Unrealized Gains	15,432,000	20,512,000
Available for Sale, Gross Unrealized Losses	1,454,000	334,000
Available for Sale, Estimated Fair Value	520,491,000	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	93,686,000	77,435,000
Held to Maturity, Gross Unrealized Gains	2,455,000	3,984,000
Held to Maturity, Gross Unrealized Losses	829,000	223,000
Total held to maturity, Estimated Fair Value	95,312,000	81,196,000
Available for Sale, Amortized Cost	27,500,000	47,512,000
Available for Sale, Gross Unrealized Gains	206,000	424,000
Available for Sale, Gross Unrealized Losses	277,000	121,000
Available for Sale, Estimated Fair Value	27,429,000	47,815,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	355,461,000	312,319,000
Held to Maturity, Gross Unrealized Gains	17,087,000	23,212,000
Held to Maturity, Gross Unrealized Losses	1,449,000	143,000
Total held to maturity, Estimated Fair Value	371,099,000	335,388,000
Available for Sale, Amortized Cost	57,221,000	66,287,000
Available for Sale, Gross Unrealized Gains	2,048,000	2,690,000
Available for Sale, Gross Unrealized Losses	67,000	12,000
Available for Sale, Estimated Fair Value	59,202,000	68,965,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	194,188,000	173,270,000
Held to Maturity, Gross Unrealized Gains	13,496,000	18,172,000
Held to Maturity, Gross Unrealized Losses	644,000	206,000
Total held to maturity, Estimated Fair Value	207,040,000	191,236,000
Available for Sale, Amortized Cost	203,519,000	202,396,000
Available for Sale, Gross Unrealized Gains	8,588,000	10,496,000
Available for Sale, Gross Unrealized Losses	365,000	184,000
Available for Sale, Estimated Fair Value	211,742,000	212,708,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	17,745,000	23,585,000
Held to Maturity, Gross Unrealized Gains	911,000	1,236,000
Held to Maturity, Gross Unrealized Losses	0	0
Total held to maturity, Estimated Fair Value	18,656,000	24,821,000
Available for Sale, Amortized Cost	218,273,000	218,763,000
Available for Sale, Gross Unrealized Gains	4,590,000	6,902,000
Available for Sale, Gross Unrealized Losses	745,000	17,000
Available for Sale, Estimated Fair Value	$ 222,118,000	$ 225,648,000

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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