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Variable Interest Entities Summary of Silverton Loan Notes (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Movement in Silverton Loan Notes [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Opening balance |  |  |  | $ 277.9 |  |  |  | $ 235.0 |  |  |  |  | $ 277.9 | $ 235.0 |
Total change in fair value for the period | $ 17.6 | $ 9.8 | $ (3.3) | (2.9) | $ (3.4) | $ (9.8) | $ 0.5 | (4.4) | $ (5.3) | $ (8.3) | $ (3.3) | $ (2.9) | 21.2 | (17.1) | $ (19.8)
Total distributed in the period |  |  |  |  |  |  |  |  |  |  |  |  | (144.2) | (108.5) |
Total issued in the period |  |  |  |  |  |  |  |  |  |  |  |  |  | 130.0 |
Closing balance as at December 31, 2017 | 107.2 |  |  |  | 277.9 |  |  |  | 235.0 |  |  |  | 107.2 | 277.9 | 235.0
Loan notes (long-term liabilities) | 54.7 |  |  |  | 142.0 |  |  |  |  |  |  |  | 54.7 | 142.0 |
Accrued expenses (current liabilities) | 52.5 |  |  |  | 135.9 |  |  |  |  |  |  |  | 52.5 | 135.9 |
Third party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Movement in Silverton Loan Notes [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Opening balance |  |  |  | 223.4 |  |  |  | 190.6 |  |  |  |  | 223.4 | 190.6 |
Total change in fair value for the period |  |  |  |  |  |  |  |  |  |  |  |  | (21.2) | 17.1 |
Total distributed in the period |  |  |  |  |  |  |  |  |  |  |  |  | (115.6) | (89.3) |
Total issued in the period |  |  |  |  |  |  |  |  |  |  |  |  |  | 105.0 |
Closing balance as at December 31, 2017 | 86.6 |  |  |  | 223.4 |  |  |  | 190.6 |  |  |  | 86.6 | 223.4 | 190.6
Loan notes (long-term liabilities) | 44.2 |  |  |  | 115.0 |  |  |  |  |  |  |  | 44.2 | 115.0 |
Accrued expenses (current liabilities) | 42.4 |  |  |  | 108.4 |  |  |  |  |  |  |  | 42.4 | 108.4 |
Aspen Holdings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Movement in Silverton Loan Notes [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Opening balance |  |  |  | $ 54.5 |  |  |  | $ 44.4 |  |  |  |  | 54.5 | 44.4 |
Total change in fair value for the period |  |  |  |  |  |  |  |  |  |  |  |  | (5.3) | 4.3 |
Total distributed in the period |  |  |  |  |  |  |  |  |  |  |  |  | (28.6) | (19.2) |
Total issued in the period |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 25.0 |
Closing balance as at December 31, 2017 | 20.6 |  |  |  | 54.5 |  |  |  | $ 44.4 |  |  |  | 20.6 | 54.5 | $ 44.4
Loan notes (long-term liabilities) | 10.5 |  |  |  | 27.0 |  |  |  |  |  |  |  | 10.5 | 27.0 |
Accrued expenses (current liabilities) | $ 10.1 |  |  |  | $ 27.5 |  |  |  |  |  |  |  | 10.1 | 27.5 |
Aspen Holdings | Third party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Movement in Silverton Loan Notes [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total change in fair value for the period |  |  |  |  |  |  |  |  |  |  |  |  | $ (26.5) | $ 21.4 |

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