Investments - Summary of Amortized Cost and Estimated Fair Values of Fixed Maturities and Equity Securities (Detail) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	$ 356,306,558	$ 336,100,948
Held to Maturity, Gross Unrealized Gains	15,170,000	11,377,000
Held to Maturity, Gross Unrealized Losses	2,062,000	2,831,000
Total held to maturity, Estimated Fair Value	369,415,000	344,647,000
Available for Sale, Amortized Cost	549,356,000	554,061,000
Available for Sale, Gross Unrealized Gains	12,346,000	12,824,000
Available for Sale, Gross Unrealized Losses	4,077,000	4,722,000
Available for Sale, Estimated Fair Value	557,625,000	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	65,104,000	61,382,000
Held to Maturity, Gross Unrealized Gains	1,454,000	1,255,000
Held to Maturity, Gross Unrealized Losses	497,000	674,000
Total held to maturity, Estimated Fair Value	66,061,000	61,963,000
Available for Sale, Amortized Cost	39,186,000	39,094,000
Available for Sale, Gross Unrealized Gains	85,000	100,000
Available for Sale, Gross Unrealized Losses	437,000	606,000
Available for Sale, Estimated Fair Value	38,834,000	38,588,000
Obligations of States and Political Subdivisions [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	133,149,000	122,793,000
Held to Maturity, Gross Unrealized Gains	10,574,000	8,404,000
Held to Maturity, Gross Unrealized Losses	195,000	369,000
Total held to maturity, Estimated Fair Value	143,528,000	130,828,000
Available for Sale, Amortized Cost	157,048,000	179,889,000
Available for Sale, Gross Unrealized Gains	5,582,000	6,637,000
Available for Sale, Gross Unrealized Losses	204,000	443,000
Available for Sale, Estimated Fair Value	162,426,000	186,083,000
Corporate Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	103,305,000	91,555,000
Held to Maturity, Gross Unrealized Gains	2,338,000	1,172,000
Held to Maturity, Gross Unrealized Losses	1,287,000	1,678,000
Total held to maturity, Estimated Fair Value	104,356,000	91,049,000
Available for Sale, Amortized Cost	93,225,000	87,715,000
Available for Sale, Gross Unrealized Gains	1,060,000	662,000
Available for Sale, Gross Unrealized Losses	442,000	921,000
Available for Sale, Estimated Fair Value	93,843,000	87,456,000
Mortgage-Backed Securities [Member]
Schedule of Equity Method Investments [Line Items]
Total held to maturity, Amortized Cost	54,749,000	60,371,000
Held to Maturity, Gross Unrealized Gains	804,000	546,000
Held to Maturity, Gross Unrealized Losses	83,000	110,000
Total held to maturity, Estimated Fair Value	55,470,000	60,807,000
Available for Sale, Amortized Cost	217,734,000	204,931,000
Available for Sale, Gross Unrealized Gains	744,000	637,000
Available for Sale, Gross Unrealized Losses	2,272,000	2,620,000
Available for Sale, Estimated Fair Value	216,206,000	202,948,000
Equity Securities [Member]
Schedule of Equity Method Investments [Line Items]
Available for Sale, Amortized Cost	42,163,000	42,432,000
Available for Sale, Gross Unrealized Gains	4,875,000	4,788,000
Available for Sale, Gross Unrealized Losses	722,000	132,000
Available for Sale, Estimated Fair Value	46,316,000	47,088,000
Fixed Maturities [Member]
Schedule of Equity Method Investments [Line Items]
Available for Sale, Amortized Cost	507,193,000	511,629,000
Available for Sale, Gross Unrealized Gains	7,471,000	8,036,000
Available for Sale, Gross Unrealized Losses	3,355,000	4,590,000
Available for Sale, Estimated Fair Value	$ 511,309,000	$ 515,075,000