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Fair Value Measurements - Reconciliation of Liabilities Using Level 3 Inputs (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
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Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016 | Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) | $ 4.7 |  | $ 4.7 |  | $ 108.4
Level 3 |  |  |  |  |
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) | 4.7 | $ 1.4 | 4.7 | $ 1.4 | $ 108.4
Level 3 | Silverton 2015 |  |  |  |  |
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance |  | 107.6 |  | 190.6 |
Distributed to third party |  | (1.6) |  | (89.0) |
Total change in fair value included in the statement of operations |  | (0.5) |  | 3.9 |
Ending balance |  | $ 105.5 |  | $ 105.5 |
Level 3 | Silverton 2016 |  |  |  |  |
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance | 115.1 |  | 223.4 |  |
Distributed to third party | (2.9) |  | (114.1) |  |
Total change in fair value included in the statement of operations | 3.3 |  | 6.2 |  |
Ending balance | $ 115.5 |  | $ 115.5 |  |

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- Definition Debt Instrument, Accrued Interest and Payables, Fair Value Disclosure + References No definition available. + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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- Definition Amount of increase (decrease) of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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