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Summary Of Investments Other Than Investments In Related Parties (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] |  |
Cost | $ 3,768,141 | $ 3,600,120
Fair value | 3,768,091 | 3,732,728
Amounts in the balance sheet | 3,768,091 | 3,732,728
US Government Agencies Debt Securities [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 25,131 | 13,355
Fair value | 25,003 | 13,236
Amounts in the balance sheet | 25,003 | 13,236
Municipal Bonds [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 2,599,056 | 2,490,150
Fair value | 2,620,132 | 2,556,532
Amounts in the balance sheet | 2,620,132 | 2,556,532
Collateralized Mortgage Backed Securities [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 30,640 | 26,454
Fair value | 30,952 | 27,165
Amounts in the balance sheet | 30,952 | 27,165
Corporate Debt Securities [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 107,479 | 136,013
Fair value | 105,524 | 137,542
Amounts in the balance sheet | 105,524 | 137,542
Collateralized Debt Obligations [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 169,626 | 104,323
Fair value | 165,789 | 105,202
Amounts in the balance sheet | 165,789 | 105,202
Other Debt Obligations [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 37,609 | 52,935
Fair value | 37,761 | 53,100
Amounts in the balance sheet | 37,761 | 53,100
Fixed Maturity Securities [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 2,969,541 | 2,823,230
Fair value | 2,985,161 | 2,892,777
Amounts in the balance sheet | 2,985,161 | 2,892,777
Common Stock [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 438,504 | 368,619
Fair value | 430,973 | 429,367
Amounts in the balance sheet | 430,973 | 429,367
Nonredeemable Preferred Stock [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 34,429 | 34,429
Fair value | 31,433 | 34,869
Amounts in the balance sheet | 31,433 | 34,869
Private Equity Funds [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 1,481 | 69,668
Fair value | 1,445 |
Amounts in the balance sheet | 1,445 | 71,523
Private Equity Funds Net Asset Value [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 69,668 | 1,481
Fair value |  | 1,481
Amounts in the balance sheet | 65,780 | 1,481
Equity Securities [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 544,082 | 474,197
Fair value | 529,631 | 537,240
Amounts in the balance sheet | 529,631 | 537,240
Short-term Investments [Member] |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Cost | 254,518 | 302,693
Fair value | 253,299 | 302,711
Amounts in the balance sheet | $ 253,299 | $ 302,711

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