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Reserves for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Insurance [Abstract]
Reserve releases	$ (92.8)	$ (78.2)	$ (129.3)
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss portfolio transfers (disposed)	(125.7)	5.7	(80.1)
Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed of or Ceded	$ 125.7		$ 85.8
AgriLogic
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss portfolio transfers (disposed)		$ 5.7

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- Definition

Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed of or Ceded

+ References

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- Definition

Unpaid claims and claims adjustment expense specialty reinsurance contracts.

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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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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-18)
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915

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