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Reinsurance - Narrative (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Plus reinsurance recoverable on unpaid losses at period end	$ 1,656.4	$ 1,515.2	$ 779.4	$ 560.7
Reinsurance recoverables, uncollateralized	$ 1,126.5	$ 1,001.9
Munich Re
Concentration Risk [Line Items]
Concentration Risk, Percentage	16.70%	17.00%
Lloyd's
Concentration Risk [Line Items]
Concentration Risk, Percentage	12.10%	13.80%
Aioi Nissay Dowa
Concentration Risk [Line Items]
Concentration Risk, Percentage	8.60%	7.60%

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- Definition

Reinsurance Recoverables, Uncollateralized

+ References

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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.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
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- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387

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-Name Accounting Standards Codification
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-Section S99
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-Subparagraph (SX 210.7-03.(a),6)
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