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Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2014 | Dec. 31, 2014 | Sep. 30, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |  |
Prior Accident Year |  |  |  | $ (4.0) | $ 92.0 | $ 0.0
Increase (decrease) in reserves |  |  |  | 708.9 | 824.0 | 622.1
Current Accident Year |  |  |  | 712.9 | 732.0 | $ 622.1
OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |  |
Prior Accident Year | $ 75.5 | $ 75.5 | $ 14.3 |  | 75.5 |
Increase (decrease) in reserves |  | 109.2 | 109.2 | 1.8 | 89.8 |
Current Accident Year |  | 33.7 | $ 33.7 |  |  |
Star & Shield Insurance Exchange [Member] |  |  |  |  |  |
Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |  |
Prior Accident Year |  |  |  | $ 2.2 | 2.2 |
Professional Insurance | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |  |
Prior Accident Year |  | 46.4 |  |  |  |
Increase (decrease) in reserves |  | 69.3 |  |  | 59.1 |
Current Accident Year |  | 22.9 |  |  |  |
Crop | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |  |
Prior Accident Year |  | 0.0 |  |  |  |
Increase (decrease) in reserves |  | 3.8 |  |  | 0.0 |
Current Accident Year |  | 3.8 |  |  |  |
Specialty Products | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |  |
Prior Accident Year |  | 51.7 |  |  |  |
Increase (decrease) in reserves |  | 80.1 |  |  | $ 61.8 |
Current Accident Year |  | 28.4 |  |  |  |
Management's estimate [Member] | Specialty Products | OneBeacon |  |  |  |  |  |
Loss and Loss Adjustment Expense Reserve Activity |  |  |  |  |  |
Current Accident Year |  | $ 29.9 |  |  |  |

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- Definition The amount of the other adjustment made to the claims reserves during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount 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 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 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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