Investments - Available-for-sale Fixed Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less, amortized cost	$ 13,283
Due after one year through five years, amortized cost	36,167
Due after five years through ten years, amortized cost	12,271
Due after ten years, amortized cost	9,649
Securities with contractual maturities, amortized cost	71,370
Fixed maturity securities, amortized cost	137,025	$ 137,004
Estimated Fair Value
Due in one year or less, fair value	13,258
Due after one year through five years, fair value	35,701
Due after five years through ten years, fair value	12,011
Due after ten years, fair value	9,507
Securities with contractual maturities, fair value	70,477
Fixed maturity securities	134,711	136,536
Collateralized mortgage obligations
Amortized Cost
Commercial mortgage and asset-backed, amortized cost	2,408
Fixed maturity securities, amortized cost	2,408	2,563
Estimated Fair Value
Commercial mortgage and asset-backed, fair value	2,365
Fixed maturity securities	2,365	2,546
Commercial mortgage-backed securities
Amortized Cost
Commercial mortgage and asset-backed, amortized cost	3,935
Fixed maturity securities, amortized cost	3,935	3,532
Estimated Fair Value
Commercial mortgage and asset-backed, fair value	3,850
Fixed maturity securities	3,850	3,491
Mortgage-backed securities
Amortized Cost
Commercial mortgage and asset-backed, amortized cost	33,981
Fixed maturity securities, amortized cost	33,981	33,942
Estimated Fair Value
Commercial mortgage and asset-backed, fair value	32,761
Fixed maturity securities	32,761	33,458
Asset-backed securities
Amortized Cost
Commercial mortgage and asset-backed, amortized cost	25,331
Fixed maturity securities, amortized cost	25,331	23,539
Estimated Fair Value
Commercial mortgage and asset-backed, fair value	25,258
Fixed maturity securities	$ 25,258	$ 23,540