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Loss and Loss Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2017 | Mar. 31, 2016 | Jun. 30, 2016
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |
Net (favorable) and unfavorable loss reserve development | $ 0.0 | $ (0.1) |
Loss and LAE incurred relating to current year losses | $ 151.7 | 161.2 |
Star & Shield Insurance Exchange |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |
Net (favorable) and unfavorable loss reserve development |  | $ (0.1) |
OneBeacon [Member] |  |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |  |
Net (favorable) and unfavorable loss reserve development |  |  | $ 0.0

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- Definition Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period 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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- 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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