SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Thousands	Dec. 31, 2021 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 6,413,496
Fair Value	6,603,649
Amount at which shown in the balance sheet	6,603,649
Fixed income
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6,397,658
Fair Value	6,587,581
Amount at which shown in the balance sheet	6,587,581
Fixed income | U.S. Treasury securities and obligations of U.S. government corporations and agencies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	133,990
Fair Value	133,407
Amount at which shown in the balance sheet	133,407
Fixed income | Obligations of U.S. states and political subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	2,408,688
Fair Value	2,534,653
Amount at which shown in the balance sheet	2,534,653
Fixed income | Foreign government debt
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	13,749
Fair Value	13,650
Amount at which shown in the balance sheet	13,650
Fixed income | Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	251,110
Fair Value	250,835
Amount at which shown in the balance sheet	250,835
Fixed income | ABS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	150,888
Fair Value	150,710
Amount at which shown in the balance sheet	150,710
Fixed income | Collateralized loan obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	360,436
Fair Value	360,939
Amount at which shown in the balance sheet	360,939
Fixed income | Mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	625,321
Fair Value	628,240
Amount at which shown in the balance sheet	628,240
Fixed income | All other corporate bonds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	2,453,476
Fair Value	2,515,147
Amount at which shown in the balance sheet	2,515,147
Equity Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	15,838
Fair Value	16,068
Amount at which shown in the balance sheet	16,068
Equity Securities | Industrial, miscellaneous and all other
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	15,838
Fair Value	16,068
Amount at which shown in the balance sheet	$ 16,068