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Fair Value Measurements - Inputs Used (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums | $ 3,360.9 | $ 3,147.0 | $ 2,997.3
Minimum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums | 50.1 | 38.9 |
Reserve for losses | 4.2 | 2.7 |
Initial value of issuance | 325.0 | 220.0 |
Maximum | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gross written premiums | 61.1 | 43.4 |
Reserve for losses | $ 61.9 | $ 11.8 |
Contract period | 365 days | 365 days |
Initial value of issuance | $ 325.0 | $ 220.0 |
Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Initial value of issuance | 44.2 | 115.0 |
Liabilities measured using level 3 inputs classified as other payables | 42.4 | 108.4 |
Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Initial value of issuance | 44.2 | 115.0 |
Liabilities measured using level 3 inputs classified as other payables | 42.4 | 108.4 |
Silverton 2015 | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of Loan Notes |  | 223.4 | $ 190.6
Silverton 2016 | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of Loan Notes | $ 86.6 | $ (223.4) |

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- Definition This item represents the amount of the total realized and unrealized gains or losses for the period which are included in the statement of income (or changes in net assets); the fair value of which assets and liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3). + 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)(1) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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