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Loss and Loss Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2016 | Mar. 31, 2015
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Equity in earnings of unconsolidated affiliates, net of tax | $ 0.0 | $ 7.3
Net (favorable) and unfavorable loss reserve development | (0.1) | (1.9)
Star & Shield Insurance Exchange |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net (favorable) and unfavorable loss reserve development | $ 0.1 | 0.1
OneBeacon [Member] |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Net (favorable) and unfavorable loss reserve development |  | $ 1.8
OneBeacon [Member] |  |
Loss and Loss Adjustment, Expense Reserves [Line Items] |  |
Percentage of Quota Share Reinsurance Agreement | 50.00% |

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- Definition This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.12) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 9 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 7 + Details
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- Definition Amount 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 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 Represents the percentage ceded under quota share reinsurance agreement. + References No definition available. + Details
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