Investments - Fixed-maturity Securities Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale Securities, Debt Maturities [Abstract]
Available-for-sale debt securities, amortized cost basis		$ 2,301,238
Available-for-sale debt securities, at fair value	$ 2,538,197	2,464,611
Fixed Maturities
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, amortized cost	208,216
Due after one year through five years, amortized cost	827,129
Due after five years through 10 years, amortized cost	649,926
Due after 10 years, amortized cost	281,779
Total fixed-maturity securities with single maturity dates, amortized cost	1,967,050
Mortgage and asset-backed securities, amortized cost	473,131
Available-for-sale debt securities, amortized cost basis	2,440,181	$ 2,301,238
Due in one year or less, fair value	211,314
Due after one year through five years, fair value	881,535
Due after five years through 10 years, fair value	671,415
Due after 10 years, fair value	286,469
Total fixed-maturity securities with single maturity dates, fair value	2,050,733
Mortgage and asset-backed securities, fair value	487,464
Available-for-sale debt securities, at fair value	$ 2,538,197