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Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Thousands | Dec. 31, 2018 USD ($)
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Amortized Cost or Cost | $ 5,948,735
Carrying Amount | 5,960,651
Short-term Investments [Member] |
Amortized Cost or Cost | 323,864
Fair Value | 323,864
Carrying Amount | 323,864
Other Investments [Member] |
Amortized Cost or Cost | 178,938
Carrying Amount | 178,938
Held-to-maturity Securities [Member] | Obligations of States and Political Subdivisions [Member] |
Amortized Cost or Cost | 17,431
Fair Value | 17,969
Carrying Amount | 17,470
Held-to-maturity Securities [Member] | Public Utilities |
Amortized Cost or Cost | 4,707
Fair Value | 5,264
Carrying Amount | 4,818
Held-to-maturity Securities [Member] | All other corporate securities |
Amortized Cost or Cost | 14,883
Fair Value | 15,084
Carrying Amount | 14,822
Held-to-maturity Securities [Member] | Fixed Income Securities [Member] |
Amortized Cost or Cost | 37,021
Fair Value | 38,317
Carrying Amount | 37,110
AFS Fixed Income Securities [Member] | US Government Agencies Debt Securities [Member] |
Amortized Cost or Cost | 120,092
Fair Value | 121,310
Carrying Amount | 121,310
AFS Fixed Income Securities [Member] | Foreign Government |
Amortized Cost or Cost | 23,202
Fair Value | 23,131
Carrying Amount | 23,131
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |
Amortized Cost or Cost | 1,121,615
Fair Value | 1,138,469
Carrying Amount | 1,138,469
AFS Fixed Income Securities [Member] | Public Utilities |
Amortized Cost or Cost | 28,464
Fair Value | 28,393
Carrying Amount | 28,393
AFS Fixed Income Securities [Member] | All other corporate securities |
Amortized Cost or Cost | 1,611,388
Fair Value | 1,589,015
Carrying Amount | 1,589,015
AFS Fixed Income Securities [Member] | CLO and Other ABS |
Amortized Cost or Cost | 720,193
Fair Value | 717,362
Carrying Amount | 717,362
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] |
Amortized Cost or Cost | 527,409
Fair Value | 527,078
Carrying Amount | 527,078
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |
Amortized Cost or Cost | 1,118,435
Fair Value | 1,128,342
Carrying Amount | 1,128,342
AFS Fixed Income Securities [Member] | Fixed Income Securities [Member] |
Amortized Cost or Cost | 5,270,798
Fair Value | 5,273,100
Carrying Amount | 5,273,100
Equity Securities [Member] | Public Utilities |
Amortized Cost or Cost | 2,416
Fair Value | 2,732
Carrying Amount | 2,732
Equity Securities [Member] | Banks, trusts, and insurance companies |
Amortized Cost or Cost | 7,448
Fair Value | 8,331
Carrying Amount | 8,331
Equity Securities [Member] | Industrial, miscellaneous and all other |
Amortized Cost or Cost | 125,361
Fair Value | 133,664
Carrying Amount | 133,664
Equity Securities [Member] | Preferred Stock [Member] |
Amortized Cost or Cost | 2,889
Fair Value | 2,912
Carrying Amount | 2,912
Equity Securities [Member] | Equity Securities [Member] |
Amortized Cost or Cost | 138,114
Fair Value | 147,639
Carrying Amount | $ 147,639

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