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Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Mar. 31, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities | $ 152,900,000 | $ 35,200,000 |  | $ (3,800,000) | $ 280,800,000 |
Trading Securities, Realized and Unrealized Gain (Loss) | 2,200,000 | 200,000 |  | (6,500,000) | 12,400,000 |
Fixed maturity investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities | 30,000,000.0 | 16,900,000 |  | 32,200,000 | 38,000,000.0 |
Short-term investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities | 0 | 300,000 |  | 400,000 | 300,000 |
Common equity securities |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities | 98,300,000 | 27,800,000 |  | (42,500,000) | 139,100,000 |
Other long-term investments |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities | 24,600,000 | (9,800,000) |  | 6,100,000 | 103,400,000 |
MediaAlpha |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Unrealized investment gain | 15,000,000.0 |  | $ 114,700,000 | 45,000,000.0 | 114,700,000 | $ 180,000,000.0
Trading Securities, Realized and Unrealized Gain (Loss) |  |  |  |  | 67,500,000 |
Investments Held [Member] |  |  |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities | $ 150,700,000 | $ 35,000,000.0 |  | $ 2,700,000 | $ 268,400,000 |

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