Investments - Summary of Amortized Cost and Estimated Fair Value of Fixed Maturities by Contractual Maturity (Detail) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Held to maturity
Due in one year or less, Amortized Cost	$ 22,709,000
Due after one year through five years, Amortized Cost	73,546,000
Due after five years through ten years, Amortized Cost	149,120,000
Due after ten years, Amortized Cost	123,551,000
Mortgage-backed securities, Amortized Cost	38,507,000
Total held to maturity, Amortized Cost	407,432,729	$ 402,798,518
Available for sale
Due in one year or less, Amortized Cost	40,681,000
Due after one year through five years, Amortized Cost	87,436,000
Due after five years through ten years, Amortized Cost	100,042,000
Due after ten years, Amortized Cost	13,804,000
Mortgage-backed securities, Amortized Cost	309,502,000
Total available for sale, Amortized Cost	551,465,000
Held to maturity
Due in one year or less, Estimated Fair Value	22,708,000
Due after one year through five years, Estimated Fair Value	75,198,000
Due after five years through ten years, Estimated Fair Value	153,022,000
Due after ten years, Estimated Fair Value	129,318,000
Mortgage-backed securities, Estimated Fair Value	38,601,000
Total held to maturity, Estimated Fair Value	418,847,000	405,038,000
Available for sale
Due in one year or less, Estimated Fair Value	40,926,000
Due after one year through five years, Estimated Fair Value	87,974,000
Due after five years through ten years, Estimated Fair Value	100,172,000
Due after ten years, Estimated Fair Value	14,086,000
Mortgage-backed securities, Estimated Fair Value	306,864,000
Total available for sale, Estimated Fair Value	$ 550,021,624	$ 526,558,304