Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities and Equity Securities (Detail) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 352,296,461	$ 336,100,948
Held to Maturity, Gross Unrealized Gains	12,218,000	11,377,000
Held to Maturity, Gross Unrealized Losses	2,218,000	2,831,000
Total held to maturity, Estimated Fair Value	362,296,000	344,647,000
Available for Sale, Amortized Cost	551,481,000	554,061,000
Available for Sale, Gross Unrealized Gains	12,893,000	12,824,000
Available for Sale, Gross Unrealized Losses	4,604,000	4,722,000
Available for Sale, Estimated Fair Value	559,770,000	562,163,000
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	63,474,000	61,382,000
Held to Maturity, Gross Unrealized Gains	1,338,000	1,255,000
Held to Maturity, Gross Unrealized Losses	658,000	674,000
Total held to maturity, Estimated Fair Value	64,154,000	61,963,000
Available for Sale, Amortized Cost	39,090,000	39,094,000
Available for Sale, Gross Unrealized Gains	93,000	100,000
Available for Sale, Gross Unrealized Losses	576,000	606,000
Available for Sale, Estimated Fair Value	38,607,000	38,588,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	129,306,000	122,793,000
Held to Maturity, Gross Unrealized Gains	8,844,000	8,404,000
Held to Maturity, Gross Unrealized Losses	285,000	369,000
Total held to maturity, Estimated Fair Value	137,865,000	130,828,000
Available for Sale, Amortized Cost	168,602,000	179,889,000
Available for Sale, Gross Unrealized Gains	6,217,000	6,637,000
Available for Sale, Gross Unrealized Losses	319,000	443,000
Available for Sale, Estimated Fair Value	174,500,000	186,083,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	102,024,000	91,555,000
Held to Maturity, Gross Unrealized Gains	1,372,000	1,172,000
Held to Maturity, Gross Unrealized Losses	1,166,000	1,678,000
Total held to maturity, Estimated Fair Value	102,230,000	91,049,000
Available for Sale, Amortized Cost	91,299,000	87,715,000
Available for Sale, Gross Unrealized Gains	762,000	662,000
Available for Sale, Gross Unrealized Losses	761,000	921,000
Available for Sale, Estimated Fair Value	91,300,000	87,456,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	57,492,000	60,371,000
Held to Maturity, Gross Unrealized Gains	664,000	546,000
Held to Maturity, Gross Unrealized Losses	109,000	110,000
Total held to maturity, Estimated Fair Value	58,047,000	60,807,000
Available for Sale, Amortized Cost	208,865,000	204,931,000
Available for Sale, Gross Unrealized Gains	645,000	637,000
Available for Sale, Gross Unrealized Losses	2,748,000	2,620,000
Available for Sale, Estimated Fair Value	206,762,000	202,948,000
Equity Securities [Member]
Schedule of Equity Method Investments [Line Items]
Available for Sale, Amortized Cost	43,625,000	42,432,000
Available for Sale, Gross Unrealized Gains	5,176,000	4,788,000
Available for Sale, Gross Unrealized Losses	200,000	132,000
Available for Sale, Estimated Fair Value	48,601,000	47,088,000
Fixed Maturities [Member]
Schedule of Equity Method Investments [Line Items]
Available for Sale, Amortized Cost	507,856,000	511,629,000
Available for Sale, Gross Unrealized Gains	7,717,000	8,036,000
Available for Sale, Gross Unrealized Losses	4,404,000	4,590,000
Available for Sale, Estimated Fair Value	$ 511,169,000	$ 515,075,000