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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

X

- 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

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+ References

No definition available.

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