Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities (Detail) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 407,432,729	$ 402,798,518
Held to Maturity, Gross Unrealized Gains	13,530,000	8,872,000
Held to Maturity, Gross Unrealized Losses	2,116,000	6,632,000
Total held to maturity, Estimated Fair Value	418,847,000	405,038,000
Available for Sale, Amortized Cost	551,465,000	535,113,000
Available for Sale, Gross Unrealized Gains	3,620,000	2,139,000
Available for Sale, Gross Unrealized Losses	5,063,000	10,694,000
Available for Sale, Estimated Fair Value	550,022,000	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	79,159,000	76,222,000
Held to Maturity, Gross Unrealized Gains	543,000	175,000
Held to Maturity, Gross Unrealized Losses	477,000	1,087,000
Total held to maturity, Estimated Fair Value	79,225,000	75,310,000
Available for Sale, Amortized Cost	30,323,000	45,188,000
Available for Sale, Gross Unrealized Gains	36,000	25,000
Available for Sale, Gross Unrealized Losses	322,000	1,003,000
Available for Sale, Estimated Fair Value	30,037,000	44,210,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	160,241,000	159,292,000
Held to Maturity, Gross Unrealized Gains	10,792,000	8,237,000
Held to Maturity, Gross Unrealized Losses	51,000	704,000
Total held to maturity, Estimated Fair Value	170,982,000	166,825,000
Available for Sale, Amortized Cost	68,077,000	73,761,000
Available for Sale, Gross Unrealized Gains	1,610,000	1,762,000
Available for Sale, Gross Unrealized Losses	116,000	307,000
Available for Sale, Estimated Fair Value	69,571,000	75,216,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	129,526,000	127,010,000
Held to Maturity, Gross Unrealized Gains	1,951,000	396,000
Held to Maturity, Gross Unrealized Losses	1,438,000	4,391,000
Total held to maturity, Estimated Fair Value	130,039,000	123,015,000
Available for Sale, Amortized Cost	143,563,000	140,689,000
Available for Sale, Gross Unrealized Gains	1,202,000	203,000
Available for Sale, Gross Unrealized Losses	1,215,000	3,059,000
Available for Sale, Estimated Fair Value	143,550,000	137,833,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	38,507,000	40,274,000
Held to Maturity, Gross Unrealized Gains	244,000	64,000
Held to Maturity, Gross Unrealized Losses	150,000	450,000
Total held to maturity, Estimated Fair Value	38,601,000	39,888,000
Available for Sale, Amortized Cost	309,502,000	275,475,000
Available for Sale, Gross Unrealized Gains	772,000	149,000
Available for Sale, Gross Unrealized Losses	3,410,000	6,325,000
Available for Sale, Estimated Fair Value	$ 306,864,000	$ 269,299,000