Investments - Schedule of Contract Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cost or Amortized Cost
One year or less	$ 33,850
After one year through five years	327,069
After five years through ten years	185,400
After ten years	127,338
Residential mortgage-backed	186,279
Commercial mortgage and asset-backed	192,941
Redeemable preferred stock	2,025
Total	1,054,902	$ 1,008,662
Fair Value
One year or less	33,743
After one year through five years	323,368
After five years through ten years	181,026
After ten years	131,208
Residential mortgage-backed	181,098
Commercial mortgage and asset-backed	190,804
Redeemable preferred stock	2,004
Total	$ 1,043,251	$ 1,016,098