SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Thousands	Dec. 31, 2019 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 5,579,738
Fair Value	5,755,220
Amount at which shown in the balance sheet	5,755,220
Fixed income
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	5,562,550
Fair Value	5,737,892
Amount at which shown in the balance sheet	5,737,892
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	195,176
Fair Value	196,203
Amount at which shown in the balance sheet	196,203
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	1,555,394
Fair Value	1,653,865
Amount at which shown in the balance sheet	1,653,865
Fixed income | Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	270,675
Fair Value	278,991
Amount at which shown in the balance sheet	278,991
Fixed income | ABS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	227,376
Fair Value	229,664
Amount at which shown in the balance sheet	229,664
Fixed income | Collateralized loan obligations
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	327,076
Fair Value	325,466
Amount at which shown in the balance sheet	325,466
Fixed income | Mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	545,618
Fair Value	547,572
Amount at which shown in the balance sheet	547,572
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	2,441,235
Fair Value	2,506,131
Amount at which shown in the balance sheet	2,506,131
Equity Securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	17,188
Fair Value	17,328
Amount at which shown in the balance sheet	17,328
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	17,188
Fair Value	17,328
Amount at which shown in the balance sheet	$ 17,328