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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 No definition available. + Details
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- Definition Unpaid claims and claims adjustment expense specialty reinsurance contracts. + References No definition available. + Details
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- Definition 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 + Details
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