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Equity-Method Eligible Investments - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions |  |  | 3 Months Ended
--- | --- | --- | ---
Mar. 23, 2021 | Oct. 30, 2020 | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha |  |  | $ (40.6) |  | $ (156.7)
Marketing Technology (MediaAlpha) |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha |  |  | $ (41.7) |  | 30.0
Secondary Offering | Marketing Technology (MediaAlpha) |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | 8,050,000.00 |  |  |  |
Sale of stock, price per share (in USD per share) | $ 46.00 | $ 19.00 |  |  |
Sale of stock, price per share, net of underwriting fees (in USD per share) | $ 44.62 | $ 17.67 |  |  |
Marketing Technology (MediaAlpha) |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Proceeds from dividends received |  | $ 55.0 |  | $ 55.0 |
Ownership interest (as a percent) | 28.50% | 35.00% |  |  |
Investment owned (in shares) | 16,900,000 | 20,500,000 |  |  |
Fully diluted ownership interest (as a percent) | 26.30% | 32.30% |  |  |
Share price (in USD per share) |  | $ 19.00 | $ 35.43 | $ 39.07 |
Investment owned, balance |  |  | $ 600.2 | $ 802.2 |
Sale of stock, price per share, net of underwriting fees (in USD per share) |  | $ 17.67 |  |  |
Marketing Technology (MediaAlpha) | IPO |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) |  | 3,600,000 |  |  |
Sale of stock, consideration received in transaction |  | $ 63.8 |  |  |
Investment owned (in shares) |  | 20,500,000 |  |  |
Marketing Technology (MediaAlpha) | Secondary Offering |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | 3,600,000 |  |  |  |
Sale of stock, consideration received in transaction | $ 160.3 |  |  |  |
Investment owned (in shares) | 16,900,000 |  |  |  |
Marketing Technology (MediaAlpha) |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Proceeds from dividends received |  |  | $ 11.4 |  | 13.7
Ownership interest (as a percent) |  |  | 28.50% | 35.00% |
Net realized and unrealized investment (losses) gains from investment in MediaAlpha |  |  | $ (41.7) |  | $ 30.0

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