Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 434,458,088	$ 402,798,518
Held to Maturity, Gross Unrealized Gains	21,067,000	8,872,000
Held to Maturity, Gross Unrealized Losses	558,000	6,632,000
Total held to maturity, Estimated Fair Value	454,967,000	405,038,000
Available for Sale, Amortized Cost	563,995,000	535,113,000
Available for Sale, Gross Unrealized Gains	7,277,000	2,139,000
Available for Sale, Gross Unrealized Losses	1,522,000	10,694,000
Available for Sale, Estimated Fair Value	569,750,000	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	87,839,000	76,222,000
Held to Maturity, Gross Unrealized Gains	1,725,000	175,000
Held to Maturity, Gross Unrealized Losses	66,000	1,087,000
Total held to maturity, Estimated Fair Value	89,498,000	75,310,000
Available for Sale, Amortized Cost	32,829,000	45,188,000
Available for Sale, Gross Unrealized Gains	85,000	25,000
Available for Sale, Gross Unrealized Losses	45,000	1,003,000
Available for Sale, Estimated Fair Value	32,869,000	44,210,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	171,174,000	159,292,000
Held to Maturity, Gross Unrealized Gains	13,282,000	8,237,000
Held to Maturity, Gross Unrealized Losses	15,000	704,000
Total held to maturity, Estimated Fair Value	184,441,000	166,825,000
Available for Sale, Amortized Cost	60,059,000	73,761,000
Available for Sale, Gross Unrealized Gains	1,856,000	1,762,000
Available for Sale, Gross Unrealized Losses	8,000	307,000
Available for Sale, Estimated Fair Value	61,907,000	75,216,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	139,037,000	127,010,000
Held to Maturity, Gross Unrealized Gains	5,513,000	396,000
Held to Maturity, Gross Unrealized Losses	438,000	4,391,000
Total held to maturity, Estimated Fair Value	144,112,000	123,015,000
Available for Sale, Amortized Cost	150,056,000	140,689,000
Available for Sale, Gross Unrealized Gains	2,998,000	203,000
Available for Sale, Gross Unrealized Losses	244,000	3,059,000
Available for Sale, Estimated Fair Value	152,810,000	137,833,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	36,408,000	40,274,000
Held to Maturity, Gross Unrealized Gains	547,000	64,000
Held to Maturity, Gross Unrealized Losses	39,000	450,000
Total held to maturity, Estimated Fair Value	36,916,000	39,888,000
Available for Sale, Amortized Cost	321,051,000	275,475,000
Available for Sale, Gross Unrealized Gains	2,338,000	149,000
Available for Sale, Gross Unrealized Losses	1,225,000	6,325,000
Available for Sale, Estimated Fair Value	$ 322,164,000	$ 269,299,000