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Investments (Components of Net Realized and Unrealized Gains (Losses)) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Gain (Loss) on Securities [Line Items] |  |  |
Unrealized Gain (Loss) on Investments recognized in income on equity securities remaining in our portfolio at end of period | $ 7,936 | $ 1,219 | $ (3,098)
Proceeds from Sale of Equity Securities | 1,320 | 137,294 | 113,339
Unrealized gains (losses) recognized in income on equity securities that were sold in the respective period | 3 | (9,868) | (26,271)
Total unrealized (losses) recognized in income on equity securities | 7,939 | (8,649) | (29,369)
Sale of fixed income securities, available-for-sale | 487,087 | $ 594,743 | $ 2,030,664
Losses on securities for which we have the intent to sell | 16,266 |  |
Corporate Securities [Member] |  |  |
Gain (Loss) on Securities [Line Items] |  |  |
Losses on securities for which we have the intent to sell | $ 12,100 |  |

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