Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 400,233,054	$ 366,655,077
Held to Maturity, Gross Unrealized Gains	7,153,000	15,342,000
Held to Maturity, Gross Unrealized Losses	9,212,000	1,547,000
Total held to maturity, Estimated Fair Value	398,174,000	380,450,000
Available for Sale, Amortized Cost	538,758,000	538,414,000
Available for Sale, Gross Unrealized Gains	1,915,000	5,418,000
Available for Sale, Gross Unrealized Losses	15,646,000	4,886,000
Available for Sale, Estimated Fair Value	525,027,000	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	76,078,000	71,736,000
Held to Maturity, Gross Unrealized Gains	5,000	804,000
Held to Maturity, Gross Unrealized Losses	2,120,000	547,000
Total held to maturity, Estimated Fair Value	73,963,000	71,993,000
Available for Sale, Amortized Cost	44,192,000	44,759,000
Available for Sale, Gross Unrealized Gains	4,000	20,000
Available for Sale, Gross Unrealized Losses	1,668,000	730,000
Available for Sale, Estimated Fair Value	42,528,000	44,049,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	156,055,000	137,581,000
Held to Maturity, Gross Unrealized Gains	6,890,000	11,162,000
Held to Maturity, Gross Unrealized Losses	1,721,000	112,000
Total held to maturity, Estimated Fair Value	161,224,000	148,631,000
Available for Sale, Amortized Cost	81,704,000	128,478,000
Available for Sale, Gross Unrealized Gains	1,696,000	3,942,000
Available for Sale, Gross Unrealized Losses	726,000	303,000
Available for Sale, Estimated Fair Value	82,674,000	132,117,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	125,879,000	108,025,000
Held to Maturity, Gross Unrealized Gains	258,000	2,860,000
Held to Maturity, Gross Unrealized Losses	4,338,000	731,000
Total held to maturity, Estimated Fair Value	121,799,000	110,154,000
Available for Sale, Amortized Cost	135,889,000	105,254,000
Available for Sale, Gross Unrealized Gains	178,000	1,011,000
Available for Sale, Gross Unrealized Losses	3,285,000	526,000
Available for Sale, Estimated Fair Value	132,782,000	105,739,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	42,221,000	49,313,000
Held to Maturity, Gross Unrealized Gains		516,000
Held to Maturity, Gross Unrealized Losses	1,033,000	157,000
Total held to maturity, Estimated Fair Value	41,188,000	49,672,000
Available for Sale, Amortized Cost	276,973,000	259,923,000
Available for Sale, Gross Unrealized Gains	37,000	445,000
Available for Sale, Gross Unrealized Losses	9,967,000	3,327,000
Available for Sale, Estimated Fair Value	$ 267,043,000	$ 257,041,000