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Segment Reporting - Summary of the Schedule of the Company's gross written premiums based on the location of the insured risk (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |
Gross written premiums | $ 3,360.9 | $ 3,147.0 | $ 2,997.3
Australia/Asia |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross written premiums | 167.3 | 140.5 | 140.0
Caribbean |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross written premiums | 17.6 | 14.3 | 20.3
Europe |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross written premiums | 94.5 | 109.7 | 113.6
United Kingdom |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross written premiums | 258.3 | 231.4 | 223.6
United States & Canada |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross written premiums | 1,729.3 | 1,597.0 | 1,479.5
Worldwide excluding United States |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross written premiums | 88.1 | 90.7 | 107.2
Worldwide including United States |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross written premiums | 868.6 | 837.2 | 793.6
Others |  |  |
Segment Reporting Information [Line Items] |  |  |
Gross written premiums | $ 137.2 | $ 126.2 | $ 119.5

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- Definition Amount of direct and assumed premiums written. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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