Investments - Fixed-maturity Securities Classified by Contractual Maturity Securities Date (Details) - USD ($) $ in Thousands	Dec. 31, 2018		Dec. 31, 2017	[2]
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, amortized cost	$ 221,508
Due after one year through five years, amortized cost	871,181
Due after five years through 10 years, amortized cost	529,167
Due after 10 years, amortized cost	52,132
Total fixed-maturity securities with single maturity dates, amortized cost	1,673,988
Mortgage and asset-backed securities, amortized cost	404,834
Available-for-sale debt securities, amortized cost basis	2,078,822	[1]	$ 1,877,326
Due in one year or less, fair value	222,803
Due after one year through five years, fair value	867,564
Due after five years through 10 years, fair value	519,647
Due after 10 years, fair value	54,555
Total fixed-maturity securities with single maturity dates, fair value	1,664,569
Mortgage and asset-backed securities, fair value	405,066
Available-for-sale debt securities, at fair value	$ 2,069,635	[1]	$ 1,927,842

[1] ^{Includes $0.1 million of OTTI losses related to corporates and mortgage- and asset-backed securities recognized in accumulated other comprehensive income (loss).}

[2] Includes $0.2 million of OTTI losses related to corporates and mortgage- and asset-backed securities recognized in accumulated other comprehensive income (loss).