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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, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross written premiums	$ 3,446.9	$ 3,360.9	$ 3,147.0
Australia/Asia
Segment Reporting Information [Line Items]
Gross written premiums	175.9	167.3	140.5
Caribbean
Segment Reporting Information [Line Items]
Gross written premiums	7.7	17.6	14.3
Europe
Segment Reporting Information [Line Items]
Gross written premiums	92.6	94.5	109.7
United Kingdom
Segment Reporting Information [Line Items]
Gross written premiums	290.1	258.3	231.4
United States & Canada
Segment Reporting Information [Line Items]
Gross written premiums	1,875.9	1,729.3	1,597.0
Worldwide excluding United States
Segment Reporting Information [Line Items]
Gross written premiums	70.1	88.1	90.7
Worldwide including United States
Segment Reporting Information [Line Items]
Gross written premiums	775.8	868.6	837.2
Others
Segment Reporting Information [Line Items]
Gross written premiums	$ 158.8	$ 137.2	$ 126.2

X

- Definition

Amount of direct and assumed premiums written.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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.

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