Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities and Equity Securities (Detail) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 328,166,897	$ 310,258,704
Held to Maturity, Gross Unrealized Gains	25,351,000	14,369,000
Held to Maturity, Gross Unrealized Losses	654,000	1,829,000
Total held to maturity, Estimated Fair Value	352,864,000	322,799,000
Available for Sale, Amortized Cost	531,251,000	524,775,000
Available for Sale, Gross Unrealized Gains	24,253,000	16,975,000
Available for Sale, Gross Unrealized Losses	788,000	3,096,000
Available for Sale, Estimated Fair Value	554,716,000	538,654,000
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	58,132,000	51,194,000
Held to Maturity, Gross Unrealized Gains	3,669,000	1,544,000
Total held to maturity, Estimated Fair Value	61,801,000	52,738,000
Available for Sale, Amortized Cost	36,652,000	37,080,000
Available for Sale, Gross Unrealized Gains	518,000	160,000
Available for Sale, Gross Unrealized Losses		51,000
Available for Sale, Estimated Fair Value	37,170,000	37,189,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	121,105,000	119,115,000
Held to Maturity, Gross Unrealized Gains	15,831,000	10,828,000
Held to Maturity, Gross Unrealized Losses		119,000
Total held to maturity, Estimated Fair Value	136,936,000	129,824,000
Available for Sale, Amortized Cost	188,653,000	223,769,000
Available for Sale, Gross Unrealized Gains	12,320,000	13,151,000
Available for Sale, Gross Unrealized Losses	21,000	364,000
Available for Sale, Estimated Fair Value	200,952,000	236,556,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	80,673,000	65,307,000
Held to Maturity, Gross Unrealized Gains	3,181,000	816,000
Held to Maturity, Gross Unrealized Losses	654,000	1,561,000
Total held to maturity, Estimated Fair Value	83,200,000	64,562,000
Available for Sale, Amortized Cost	85,167,000	73,474,000
Available for Sale, Gross Unrealized Gains	1,930,000	350,000
Available for Sale, Gross Unrealized Losses	100,000	1,012,000
Available for Sale, Estimated Fair Value	86,997,000	72,812,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	68,257,000	74,643,000
Held to Maturity, Gross Unrealized Gains	2,670,000	1,181,000
Held to Maturity, Gross Unrealized Losses		149,000
Total held to maturity, Estimated Fair Value	70,927,000	75,675,000
Available for Sale, Amortized Cost	181,222,000	154,687,000
Available for Sale, Gross Unrealized Gains	3,778,000	1,045,000
Available for Sale, Gross Unrealized Losses	10,000	896,000
Available for Sale, Estimated Fair Value	184,990,000	154,836,000
Equity Securities [Member]
Schedule of Equity Method Investments [Line Items]
Available for Sale, Amortized Cost	39,557,000	35,765,000
Available for Sale, Gross Unrealized Gains	5,707,000	2,269,000
Available for Sale, Gross Unrealized Losses	657,000	773,000
Available for Sale, Estimated Fair Value	44,607,000	37,261,000
Fixed Maturities [Member]
Schedule of Equity Method Investments [Line Items]
Available for Sale, Amortized Cost	491,694,000	489,010,000
Available for Sale, Gross Unrealized Gains	18,546,000	14,706,000
Available for Sale, Gross Unrealized Losses	131,000	2,323,000
Available for Sale, Estimated Fair Value	$ 510,109,000	$ 501,393,000