SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 4,953,501
Fair Value	4,990,561
Amount at which shown in the balance sheet	4,990,561
Fixed income
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	4,946,278
Fair Value	4,983,315
Amount at which shown in the balance sheet	4,983,315
Fixed income | U.S. Treasury securities and obligations of U.S. government corporations and agencies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	179,850
Fair Value	178,846
Amount at which shown in the balance sheet	178,846
Fixed income | Obligations of U.S. states and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,105,063
Fair Value	2,152,524
Amount at which shown in the balance sheet	2,152,524
Fixed income | Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	222,619
Fair Value	222,806
Amount at which shown in the balance sheet	222,806
Fixed income | ABS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	4,925
Fair Value	4,923
Amount at which shown in the balance sheet	4,923
Fixed income | Collateralized loan obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	100,798
Fair Value	101,023
Amount at which shown in the balance sheet	101,023
Fixed income | Mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	490,167
Fair Value	479,161
Amount at which shown in the balance sheet	479,161
Fixed income | All other corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,842,856
Fair Value	1,844,032
Amount at which shown in the balance sheet	1,844,032
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	7,223
Fair Value	7,246
Amount at which shown in the balance sheet	7,246
Equity Securities | Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	7,223
Fair Value	7,246
Amount at which shown in the balance sheet	$ 7,246