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, 2019	Dec. 31, 2018
Investment [Line Items]
Within one year, Amortized Cost	$ 49,966
After one year through five years, Amortized Cost	158,125
After five years through ten years, Amortized Cost	46,069
After ten years, Amortized Cost	179,053
Totals, Amortized Cost	445,117	$ 479,772
Within one year, Fair Value	50,018
After one year through five years, Fair Value	159,944
After five years through ten years, Fair Value	47,983
After ten years, Fair Value	188,096
Total, Fair Value	457,893	478,730
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale securities, Amortized Cost	11,904
Totals, Amortized Cost	11,904	12,835
Available-for-sale securities, Fair value	11,852
Total, Fair Value	$ 11,852	$ 12,515