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Fair Value Measurements - Additional Information (Details) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2019 USD ($) $ / Investment | Dec. 31, 2018 USD ($) $ / Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Quotes per fixed income investment | $ / Investment |  |  |  | 2.4 | 2.2
Silverton 2015 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period |  |  | $ (85,000,000) |  |
Silverton 2015, Third-party Funded |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period |  |  | $ (70,000,000) |  |
Silverton 2016 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period |  | $ (125,000,000) |  |  |
Silverton 2016, Third-party Funded |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period |  | $ (100,000,000) |  |  |
Silverton 2017 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period | $ (130,000,000) |  |  |  |
Silverton 2017, Third-party Funded |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period |  |  |  |  | $ (105,000,000)
Recurring | Level 3 | Silverton |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Repayments of debt |  |  |  | $ (7,700,000) |
Recurring | Level 3 | Silverton 2016 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Level 3 liabilities settled |  |  |  |  | 86,400,000
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 |  |  |  | 279,700,000 |
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 |  |  |  | $ 279,700,000 | $ 0

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