Investments - Summary of Amortized Cost and Estimated Fair Value of Fixed Maturities by Contractual Maturity (Detail) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Held to maturity
Due in one year or less, Amortized Cost	$ 36,112,000
Due after one year through five years, Amortized Cost	82,968,000
Due after five years through ten years, Amortized Cost	232,949,000
Due after ten years, Amortized Cost	291,306,000
Mortgage-backed securities, Amortized Cost	17,745,000
Total held to maturity, Amortized Cost	661,080,363	$ 586,609,439
Available for sale
Due in one year or less, Amortized Cost	22,414,000
Due after one year through five years, Amortized Cost	120,648,000
Due after five years through ten years, Amortized Cost	113,678,000
Due after ten years, Amortized Cost	31,500,000
Mortgage-backed securities, Amortized Cost	218,273,000
Total available for sale, Amortized Cost	506,513,000
Held to maturity
Due in one year or less, Estimated Fair Value	36,838,000
Due after one year through five years, Estimated Fair Value	87,988,000
Due after five years through ten years, Estimated Fair Value	244,151,000
Due after ten years, Estimated Fair Value	304,474,000
Mortgage-backed securities, Estimated Fair Value	18,656,000
Total held to maturity, Estimated Fair Value	692,107,000	632,641,000
Available for sale
Due in one year or less, Estimated Fair Value	22,809,000
Due after one year through five years, Estimated Fair Value	126,744,000
Due after five years through ten years, Estimated Fair Value	116,556,000
Due after ten years, Estimated Fair Value	32,264,000
Mortgage-backed securities, Estimated Fair Value	222,118,000
Total available for sale, Estimated Fair Value	$ 520,490,593	$ 555,136,017