SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 4,724,356
Fair Value	4,692,350
Amount at which shown in the balance sheet	4,692,350
Fixed income
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	4,717,212
Fair Value	4,685,222
Amount at which shown in the balance sheet	4,685,222
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	73,847
Fair Value	73,530
Amount at which shown in the balance sheet	73,530
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	2,147,458
Fair Value	2,143,016
Amount at which shown in the balance sheet	2,143,016
Fixed income | Foreign governments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	0
Fair Value	0
Amount at which shown in the balance sheet	0
Fixed income | Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	176,184
Fair Value	175,084
Amount at which shown in the balance sheet	175,084
Fixed income | ABS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	59,519
Fair Value	59,565
Amount at which shown in the balance sheet	59,565
Fixed income | Collateralized loan obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	121,151
Fair Value	121,174
Amount at which shown in the balance sheet	121,174
Fixed income | Mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	558,775
Fair Value	544,026
Amount at which shown in the balance sheet	544,026
Fixed income | All other corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,580,278
Fair Value	1,568,827
Amount at which shown in the balance sheet	1,568,827
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	7,144
Fair Value	7,128
Amount at which shown in the balance sheet	7,128
Equity Securities | Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	7,144
Fair Value	7,128
Amount at which shown in the balance sheet	$ 7,128