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Fair Value Measurements - Loan Notes Issued by Variable Interest Entities (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Gross written premiums | $ 1,116.8 | $ 998.0 |  |
Maximum | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Gross written premiums | 61.1 |  | $ 61.1 |
Reserve for losses | $ 64.5 |  | $ 61.9 |
Contract period | 365 days |  | 365 days |
Loan notes held by third parties | $ 325.0 |  | $ 325.0 |
Minimum | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Gross written premiums | 50.1 |  | 50.1 |
Reserve for losses | 4.2 |  | 4.2 |
Loan notes held by third parties | 325.0 |  | 325.0 |
Fair Value, Measurements, Recurring |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total | 7,139.7 |  | 7,485.4 |
Loan notes held by third parties | 32.2 |  | 44.2 |
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) | (7.6) |  | (42.4) |
Fair Value, Measurements, Recurring | Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Total | (39.8) |  | (86.6) |
Loan notes held by third parties | 32.2 |  | 44.2 |
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) | (7.6) |  | (42.4) |
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value |  | 115.1 |  | $ 223.4
Total |  |  | 86.6 |
Amount of Level 3 liabilities settled |  | 111.2 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) |  | $ 2.9 |  |
Fair Value, Measurements, Recurring | Level 3 | Silverton 2017 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value | 39.8 |  | $ 86.6 |
Total | 39.8 |  |  |
Amount of Level 3 liabilities settled | 45.8 |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) | $ (1.0) |  |  |

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- Definition Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 + 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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