Investments - Summary of Amortized Cost and Estimated Fair Value of Fixed Maturities by Contractual Maturity (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Held to maturity
Due in one year or less, Amortized Cost	$ 6,681,000
Due after one year through five years, Amortized Cost	52,418,000
Due after five years through ten years, Amortized Cost	139,103,000
Due after ten years, Amortized Cost	115,165,000
Mortgage-backed securities, Amortized Cost	51,910,000
Total held to maturity, Amortized Cost	365,277,271	$ 336,100,948
Available for sale
Due in one year or less, Amortized Cost	54,276,000
Due after one year through five years, Amortized Cost	94,106,000
Due after five years through ten years, Amortized Cost	110,187,000
Due after ten years, Amortized Cost	24,245,000
Mortgage-backed securities, Amortized Cost	246,925,000
Total available for sale, Amortized Cost	529,739,000
Held to maturity
Due in one year or less, Estimated Fair Value	6,689,000
Due after one year through five years, Estimated Fair Value	53,656,000
Due after five years through ten years, Estimated Fair Value	143,566,000
Due after ten years, Estimated Fair Value	122,867,000
Mortgage-backed securities, Estimated Fair Value	52,651,000
Total held to maturity, Estimated Fair Value	379,429,000	344,647,000
Available for sale
Due in one year or less, Estimated Fair Value	55,289,000
Due after one year through five years, Estimated Fair Value	95,647,000
Due after five years through ten years, Estimated Fair Value	111,712,000
Due after ten years, Estimated Fair Value	25,215,000
Mortgage-backed securities, Estimated Fair Value	245,598,000
Total available for sale, Estimated Fair Value	$ 533,461,436	$ 515,074,940