SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Thousands	Dec. 31, 2023 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 6,077,047
Fair Value	5,737,884
Amount at which shown in the balance sheet	5,737,884
Fixed income
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6,061,022
Fair Value	5,723,113
Amount at which shown in the balance sheet	5,723,113
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	167,995
Fair Value	161,682
Amount at which shown in the balance sheet	161,682
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	2,092,754
Fair Value	1,908,078
Amount at which shown in the balance sheet	1,908,078
Fixed income | Foreign government debt
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	4,486
Fair Value	3,843
Amount at which shown in the balance sheet	3,843
Fixed income | Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	251,151
Fair Value	250,952
Amount at which shown in the balance sheet	250,952
Fixed income | ABS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	173,256
Fair Value	171,273
Amount at which shown in the balance sheet	171,273
Fixed income | CLOs
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	327,467
Fair Value	326,096
Amount at which shown in the balance sheet	326,096
Fixed income | Mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	640,336
Fair Value	608,230
Amount at which shown in the balance sheet	608,230
Fixed income | All other corporate debt securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	2,375,250
Fair Value	2,264,629
Amount at which shown in the balance sheet	2,264,629
Fixed income | Commercial paper
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	28,327
Fair Value	28,330
Amount at which shown in the balance sheet	28,330
Equity Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	16,025
Fair Value	14,771
Amount at which shown in the balance sheet	14,771
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	16,025
Fair Value	14,771
Amount at which shown in the balance sheet	$ 14,771