Investments - Summary of Amortized Cost and Estimated Fair Value of Fixed Maturities by Contractual Maturity (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Held to maturity
Due in one year or less, Amortized Cost	$ 8,874,000
Due after one year through five years, Amortized Cost	35,983,000
Due after five years through ten years, Amortized Cost	79,535,000
Due after ten years, Amortized Cost	94,094,000
Mortgage-backed securities, Amortized Cost	86,520,000
Total held to maturity, Amortized Cost	305,005,882	307,391,699
Due in one year or less, Estimated Fair Value	8,888,000
Due after one year through five years, Estimated Fair Value	36,854,000
Due after five years through ten years, Estimated Fair Value	83,680,000
Due after ten years, Estimated Fair Value	103,237,000
Mortgage-backed securities, Estimated Fair Value	89,049,000
Total held to maturity, Estimated Fair Value	321,708,000	322,155,000
Available for sale
Due in one year or less, Amortized Cost	35,794,000
Due after one year through five years, Amortized Cost	85,681,000
Due after five years through ten years, Amortized Cost	112,524,000
Due after ten years, Amortized Cost	102,183,000
Mortgage-backed securities, Amortized Cost	103,971,000
Total available for sale, Amortized Cost	440,153,000
Due in one year or less, Estimated Fair Value	36,672,000
Due after one year through five years, Estimated Fair Value	89,583,000
Due after five years through ten years, Estimated Fair Value	119,343,000
Due after ten years, Estimated Fair Value	108,728,000
Mortgage-backed securities, Estimated Fair Value	106,296,000
Total available for sale, Estimated Fair Value	$ 460,621,748	$ 435,149,784