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Fair Value Measurements - Additional Information - Narrative (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 USD ($) quotesPerInvestment | Dec. 31, 2019 USD ($) quotesPerInvestment | Sep. 30, 2019 USD ($) | Mar. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Quotes per fixed income investment | quotesPerInvestment | 2.6 | 2.4 |  |
Liabilities Under Derivative Contracts | Interest rate swap | Not Designated as Hedging Instrument |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Derivative, Notional Amount | $ 0.0 | $ 1,800.0 | $ 1,800.0 | $ 3,138.0
Recurring | Level 3 | Silverton 2016 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Level 3 liabilities settled |  | 3.1 |  |
Assets | Recurring | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Transfers in/(out) | 8.3 | 0.0 |  |
Settlements and sales | (64.3) | (23.9) |  |
Held for trading financial assets, at fair value | Recurring |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Privately-held Investments, at fair value | 299.3 | 279.7 |  |
Held for trading financial assets, at fair value | Recurring | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Privately-held Investments, at fair value | $ 299.3 | $ 279.7 |  |

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