Investments - Fixed-maturity Securities Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, amortized cost	$ 161,768
Due after one year through five years, amortized cost	853,995
Due after five years through 10 years, amortized cost	498,631
Due after 10 years, amortized cost	205,366
Total fixed-maturity securities with single maturity dates, amortized cost	1,719,760
Mortgage and asset-backed securities, amortized cost	517,714
Available-for-sale debt securities, amortized cost basis	2,237,474	$ 2,274,770
Available-for-sale debt securities, at fair value	2,357,154	2,356,996
Fixed Maturities
Available-for-sale Securities, Debt Maturities [Abstract]
Available-for-sale debt securities, amortized cost basis	2,237,474	$ 2,274,770
Due in one year or less, fair value	163,796
Due after one year through five years, fair value	901,122
Due after five years through 10 years, fair value	535,477
Due after 10 years, fair value	222,465
Total fixed-maturity securities with single maturity dates, fair value	1,822,860
Mortgage and asset-backed securities, fair value	534,294
Available-for-sale debt securities, at fair value	$ 2,357,154