Investments - Schedule of Contract Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Cost or Amortized Cost
One year or less	$ 117,434
After one year through five years	253,240
After five years through ten years	192,269
After ten years	124,742
Residential mortgage-backed	140,053
Commercial mortgage and asset-backed	176,952
Redeemable preferred stock	2,025
Total	1,006,715	$ 940,212
Fair Value
One year or less	117,483
After one year through five years	255,514
After five years through ten years	193,760
After ten years	131,482
Residential mortgage-backed	139,057
Commercial mortgage and asset-backed	177,447
Redeemable preferred stock	2,075
Total	$ 1,016,818	$ 941,077