Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Within one year, Amortized Cost	$ 69,793
After one year through five years, Amortized Cost	158,974
After five years through ten years, Amortized Cost	32,366
After ten years, Amortized Cost	168,495
Fixed maturity, Cost or Amortized Cost	443,358	$ 461,236
Within one year, Fair Value	69,608
After one year through five years, Fair Value	156,733
After five years through ten years, Fair Value	31,784
After ten years, Fair Value	169,275
Total, Fair Value	440,762	465,594
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost	13,730
Fixed maturity, Cost or Amortized Cost	13,730	18,813
Available-for-sale securities, Fair value	13,362
Total, Fair Value	$ 13,362	$ 18,014