SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Thousands	Dec. 31, 2020 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 6,334,686
Fair Value	6,679,811
Amount at which shown in the balance sheet	6,679,811
Fixed income
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6,317,164
Fair Value	6,661,596
Amount at which shown in the balance sheet	6,661,596
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	264,531
Fair Value	265,693
Amount at which shown in the balance sheet	265,693
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	2,083,568
Fair Value	2,249,869
Amount at which shown in the balance sheet	2,249,869
Fixed income | Debt securities issued by foreign sovereign governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	4,485
Fair Value	4,709
Amount at which shown in the balance sheet	4,709
Fixed income | Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	303,285
Fair Value	303,011
Amount at which shown in the balance sheet	303,011
Fixed income | ABS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	203,807
Fair Value	206,686
Amount at which shown in the balance sheet	206,686
Fixed income | Collateralized loan obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	310,616
Fair Value	310,490
Amount at which shown in the balance sheet	310,490
Fixed income | Mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	738,104
Fair Value	758,668
Amount at which shown in the balance sheet	758,668
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	2,387,575
Fair Value	2,541,277
Amount at which shown in the balance sheet	2,541,277
Fixed income | Other investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	21,193
Fair Value	21,193
Amount at which shown in the balance sheet	21,193
Equity Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	17,522
Fair Value	18,215
Amount at which shown in the balance sheet	18,215
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	17,522
Fair Value	18,215
Amount at which shown in the balance sheet	$ 18,215