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Variable Interest Entities - Summary of Silverton Loan Notes (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Movement in Silverton Loan Notes [Roll Forward] |  |  |  |  |  |
Total liability balance of Loan Notes, Beginning Balance |  |  | $ 107.2 | $ 277.9 |  |
Total change in fair value for the period | $ (1.7) | $ 9.8 | (4.1) | 3.6 |  |
Total distributed in the period |  |  | (97.1) | (144.7) |  |
Total liability balance of Loan Notes, Ending Balance | 15.2 | 128.7 | 15.2 | 128.7 |  |
Loan notes (long-term liabilities) |  |  |  |  | $ 0.0 | $ 125.7
Accrued expenses (current liabilities) |  |  |  |  | 15.2 | 3.0
Total aggregate unpaid balance | 15.2 | 128.7 | 107.2 | 277.9 | 15.2 | 128.7
Third party |  |  |  |  |  |
Movement in Silverton Loan Notes [Roll Forward] |  |  |  |  |  |
Total liability balance of Loan Notes, Beginning Balance |  |  | 86.6 | 223.4 |  |
Total change in fair value for the period |  |  | 4.1 | (3.6) |  |
Total distributed in the period |  |  | (78.4) | (115.9) |  |
Total liability balance of Loan Notes, Ending Balance | 12.3 | 103.9 | 12.3 | 103.9 |  |
Loan notes (long-term liabilities) |  |  |  |  | 0.0 | 101.5
Accrued expenses (current liabilities) |  |  |  |  | 12.3 | 2.4
Total aggregate unpaid balance | 12.3 | 103.9 | 86.6 | 223.4 | 12.3 | 103.9
Aspen Holdings |  |  |  |  |  |
Movement in Silverton Loan Notes [Roll Forward] |  |  |  |  |  |
Total liability balance of Loan Notes, Beginning Balance |  |  | 20.6 | 54.5 |  |
Total change in fair value for the period |  |  | 1.0 | (0.9) |  |
Total distributed in the period |  |  | (18.7) | (28.8) |  |
Total liability balance of Loan Notes, Ending Balance | 2.9 | 24.8 | 2.9 | 24.8 |  |
Loan notes (long-term liabilities) |  |  |  |  | 0.0 | 24.2
Accrued expenses (current liabilities) |  |  |  |  | 2.9 | 0.6
Total aggregate unpaid balance | $ 2.9 | $ 24.8 | 20.6 | 54.5 | $ 2.9 | $ 24.8
Aspen Holdings | Third party |  |  |  |  |  |
Movement in Silverton Loan Notes [Roll Forward] |  |  |  |  |  |
Total change in fair value for the period |  |  | $ 5.1 | $ (4.5) |  |

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