Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities (Detail) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 458,888,742	$ 402,798,518
Held to Maturity, Gross Unrealized Gains	26,364,000	8,872,000
Held to Maturity, Gross Unrealized Losses	511,000	6,632,000
Total held to maturity, Estimated Fair Value	484,742,000	405,038,000
Available for Sale, Amortized Cost	550,792,000	535,113,000
Available for Sale, Gross Unrealized Gains	9,315,000	2,139,000
Available for Sale, Gross Unrealized Losses	1,006,000	10,694,000
Available for Sale, Estimated Fair Value	559,101,000	526,558,000
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	91,485,000	76,222,000
Held to Maturity, Gross Unrealized Gains	2,336,000	175,000
Held to Maturity, Gross Unrealized Losses	47,000	1,087,000
Total held to maturity, Estimated Fair Value	93,774,000	75,310,000
Available for Sale, Amortized Cost	19,325,000	45,188,000
Available for Sale, Gross Unrealized Gains	128,000	25,000
Available for Sale, Gross Unrealized Losses	7,000	1,003,000
Available for Sale, Estimated Fair Value	19,446,000	44,210,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	181,575,000	159,292,000
Held to Maturity, Gross Unrealized Gains	15,124,000	8,237,000
Held to Maturity, Gross Unrealized Losses	86,000	704,000
Total held to maturity, Estimated Fair Value	196,613,000	166,825,000
Available for Sale, Amortized Cost	57,178,000	73,761,000
Available for Sale, Gross Unrealized Gains	1,990,000	1,762,000
Available for Sale, Gross Unrealized Losses	3,000	307,000
Available for Sale, Estimated Fair Value	59,165,000	75,216,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	151,654,000	127,010,000
Held to Maturity, Gross Unrealized Gains	8,281,000	396,000
Held to Maturity, Gross Unrealized Losses	353,000	4,391,000
Total held to maturity, Estimated Fair Value	159,582,000	123,015,000
Available for Sale, Amortized Cost	146,644,000	140,689,000
Available for Sale, Gross Unrealized Gains	4,295,000	203,000
Available for Sale, Gross Unrealized Losses	158,000	3,059,000
Available for Sale, Estimated Fair Value	150,781,000	137,833,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	34,175,000	40,274,000
Held to Maturity, Gross Unrealized Gains	623,000	64,000
Held to Maturity, Gross Unrealized Losses	25,000	450,000
Total held to maturity, Estimated Fair Value	34,773,000	39,888,000
Available for Sale, Amortized Cost	327,645,000	275,475,000
Available for Sale, Gross Unrealized Gains	2,902,000	149,000
Available for Sale, Gross Unrealized Losses	838,000	6,325,000
Available for Sale, Estimated Fair Value	$ 329,709,000	$ 269,299,000

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 of accumulated unrecognized gain on 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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