Investments - Summary of Amortized Cost and Estimated Fair Value of Fixed Maturities by Contractual Maturity (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Held to maturity
Due in one year or less, Amortized Cost	$ 23,975,000
Due after one year through five years, Amortized Cost	72,869,000
Due after five years through ten years, Amortized Cost	166,809,000
Due after ten years, Amortized Cost	134,397,000
Mortgage-backed securities, Amortized Cost	36,408,000
Total held to maturity, Amortized Cost	434,458,088	$ 402,798,518
Available for sale
Due in one year or less, Amortized Cost	30,622,000
Due after one year through five years, Amortized Cost	91,216,000
Due after five years through ten years, Amortized Cost	106,807,000
Due after ten years, Amortized Cost	14,299,000
Mortgage-backed securities, Amortized Cost	321,051,000
Total available for sale, Amortized Cost	563,995,000
Held to maturity
Due in one year or less, Estimated Fair Value	24,103,000
Due after one year through five years, Estimated Fair Value	75,460,000
Due after five years through ten years, Estimated Fair Value	174,481,000
Due after ten years, Estimated Fair Value	144,007,000
Mortgage-backed securities, Estimated Fair Value	36,916,000
Total held to maturity, Estimated Fair Value	454,967,000	405,038,000
Available for sale
Due in one year or less, Estimated Fair Value	30,840,000
Due after one year through five years, Estimated Fair Value	93,077,000
Due after five years through ten years, Estimated Fair Value	108,931,000
Due after ten years, Estimated Fair Value	14,738,000
Mortgage-backed securities, Estimated Fair Value	322,164,000
Total available for sale, Estimated Fair Value	$ 569,749,979	$ 526,558,304