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Schedule I - Summary Of Investments (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties, Cost | $ 3,813,996 |
Investments | 3,937,223 | $ 5,740,791
Other Investments and Securities, at Cost | 362,847 |
Other Investments | 391,217 | $ 431,630
U.S. Government Obligations [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Available-for-sale Securities | 94,394 |
Short-term Investments [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 430,405 |
Available-for-sale Securities | 430,331 |
U S Government Obligations [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties, Cost | 93,636 |
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 882,631 |
Available-for-sale Securities | 879,919 |
Fixed Income Securities [Member] | Corporate Debt Securities [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 1,288,882 |
Available-for-sale Securities | 1,278,122 |
Fixed Income Securities [Member] | Debt Security, Government, Non-US [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 30,496 |
Available-for-sale Securities | 30,834 |
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 221,825 |
Available-for-sale Securities | 258,607 |
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 133,075 |
Available-for-sale Securities | 131,356 |
Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 370,199 |
Available-for-sale Securities | $ 442,443 |

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