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Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Net realized investment gains (losses), pre-tax (1) |  | $ 96,600,000 | $ (12,900,000) | $ 24,100,000
Net unrealized investment gains (losses), pre-tax (2) |  | 336,600,000 | (95,400,000) | 109,200,000
Net realized and unrealized investment gains (losses), pre-tax |  | 433,200,000 | (108,300,000) | 133,300,000
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses) |  | (87,000,000.0) | 18,200,000 | (12,900,000)
Net realized and unrealized investment gains (losses), after-tax |  | 346,200,000 | $ (90,100,000) | $ 120,400,000
MediaAlpha |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Realized gain from transaction | $ 67,500,000 | 67,500,000 |  |
Unrealized investment gain | $ 114,700,000 | $ 180,000,000.0 |  |

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- Definition The sum of the realized and unrealized net gain (loss) on trading securities during the period. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Represents the equity in unrealized gains or losses from investments in unconsolidated affiliates. + References No definition available. + Details
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- Definition Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax + References No definition available. + Details
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- Definition The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. + References No definition available. + Details
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