Investments - Unrealized Gains on Equity Securities (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | |||
|---|---|---|---|---|---|
Sep. 30, 2022 |
Sep. 30, 2021 |
Sep. 30, 2022 |
Sep. 30, 2021 |
Dec. 31, 2021 |
|
| Investments, Debt and Equity Securities [Abstract] | |||||
| Net (losses) gains recognized on equity securities | $ (5.2) | $ 95.1 | $ (102.4) | $ 100.6 | |
| Less: Net realized gains related to sales of equity securities | (0.2) | 1.1 | 20.0 | 2.1 | |
| Total fair value changes to equity securities held | (5.0) | $ 94.0 | (122.4) | $ 98.5 | |
| Gain (Loss) on Securities [Line Items] | |||||
| Equity securities at fair value | 302.6 | 302.6 | $ 445.7 | ||
| Four Equity Positions that Went Public | |||||
| Gain (Loss) on Securities [Line Items] | |||||
| Equity securities, unrealized gain | 0.6 | 78.5 | |||
| Equity securities at fair value | $ 24.4 | $ 24.4 | |||