Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Within one year, Amortized Cost	$ 73,298
After one year through five years, Amortized Cost	165,853
After five years through ten years, Amortized Cost	44,355
After ten years, Amortized Cost	176,553
Fixed maturity, Cost or Amortized Cost	473,315	$ 461,236
Within one year, Fair Value	73,137
After one year through five years, Fair Value	163,342
After five years through ten years, Fair Value	43,378
After ten years, Fair Value	175,382
Total, Fair Value	468,019	465,594
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost	13,256
Fixed maturity, Cost or Amortized Cost	13,256	18,813
Available-for-sale securities, Fair value	12,780
Total, Fair Value	$ 12,780	$ 18,014