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Fair Value Measurements - Additional Information (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 USD ($) $ / Investment | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2017 USD ($) $ / Investment | Dec. 31, 2016 USD ($) $ / Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Quotes per fixed income investment | $ / Investment | 2.1 |  |  | 2.0 | 2.1
Quotes per equity investment | $ / Investment | 3.9 |  |  | 4.0 | 3.9
Loan notes issued during the period |  |  |  |  | $ (130.0)
Silverton 2015 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period |  |  | $ (85.0) |  |
Silverton 2016 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period |  | $ (125.0) |  |  |
Silverton 2016, Third-party Funded [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period |  | (100.0) |  |  |
Silverton 2017 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period | $ (130.0) |  |  | $ 0.0 |
Silverton 2017, Third-party Funded [Member] |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Loan notes issued during the period |  | $ (105.0) |  | 0.0 |
Recurring | Level 3 | Silverton |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Repayments of debt |  |  |  | (115.6) |
Recurring | Level 3 | Silverton 2015 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Repayments of debt |  |  |  |  | (89.3)
Loan notes issued during the period |  |  |  |  | (105.0)
Recurring | Level 3 | Silverton 2016 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Repayments of debt |  |  |  | (115.6) |
Loan notes issued during the period |  |  |  | $ 0.0 |
Non-U.S government |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount | $ 83.9 |  |  |  | $ 83.9

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