Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 385,822,214	$ 366,655,077
Held to Maturity, Gross Unrealized Gains	8,543,000	15,342,000
Held to Maturity, Gross Unrealized Losses	6,832,000	1,547,000
Total held to maturity, Estimated Fair Value	387,533,000	380,450,000
Available for Sale, Amortized Cost	530,724,000	538,414,000
Available for Sale, Gross Unrealized Gains	2,527,000	5,418,000
Available for Sale, Gross Unrealized Losses	13,272,000	4,886,000
Available for Sale, Estimated Fair Value	519,979,000	538,946,000
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	72,936,000	71,736,000
Held to Maturity, Gross Unrealized Gains	40,000	804,000
Held to Maturity, Gross Unrealized Losses	1,529,000	547,000
Total held to maturity, Estimated Fair Value	71,447,000	71,993,000
Available for Sale, Amortized Cost	44,748,000	44,759,000
Available for Sale, Gross Unrealized Gains	6,000	20,000
Available for Sale, Gross Unrealized Losses	1,394,000	730,000
Available for Sale, Estimated Fair Value	43,360,000	44,049,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	149,759,000	137,581,000
Held to Maturity, Gross Unrealized Gains	8,092,000	11,162,000
Held to Maturity, Gross Unrealized Losses	795,000	112,000
Total held to maturity, Estimated Fair Value	157,056,000	148,631,000
Available for Sale, Amortized Cost	96,256,000	128,478,000
Available for Sale, Gross Unrealized Gains	2,274,000	3,942,000
Available for Sale, Gross Unrealized Losses	596,000	303,000
Available for Sale, Estimated Fair Value	97,934,000	132,117,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	118,611,000	108,025,000
Held to Maturity, Gross Unrealized Gains	360,000	2,860,000
Held to Maturity, Gross Unrealized Losses	3,746,000	731,000
Total held to maturity, Estimated Fair Value	115,225,000	110,154,000
Available for Sale, Amortized Cost	121,776,000	105,254,000
Available for Sale, Gross Unrealized Gains	165,000	1,011,000
Available for Sale, Gross Unrealized Losses	2,916,000	526,000
Available for Sale, Estimated Fair Value	119,025,000	105,739,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	44,516,000	49,313,000
Held to Maturity, Gross Unrealized Gains	51,000	516,000
Held to Maturity, Gross Unrealized Losses	762,000	157,000
Total held to maturity, Estimated Fair Value	43,805,000	49,672,000
Available for Sale, Amortized Cost	267,944,000	259,923,000
Available for Sale, Gross Unrealized Gains	82,000	445,000
Available for Sale, Gross Unrealized Losses	8,366,000	3,327,000
Available for Sale, Estimated Fair Value	$ 259,660,000	$ 257,041,000