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Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended |  | 12 Months Ended | 1 Months Ended | 12 Months Ended |  |  |  | 3 Months Ended | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  |  |  | 12 Months Ended | 3 Months Ended
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Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2013 Discontinued Operations | Dec. 31, 2012 Discontinued Operations | Dec. 31, 2011 Discontinued Operations | Oct. 31, 2011 Esurance | Dec. 31, 2012 AutoOne Discontinued Operations | Dec. 31, 2011 AutoOne Discontinued Operations | Feb. 22, 2012 AutoOne Discontinued Operations Entity | Dec. 31, 2013 OneBeacon Runoff Contract | Dec. 31, 2012 OneBeacon Runoff | Dec. 31, 2013 OneBeacon Runoff Discontinued Operations Rating | Jun. 30, 2013 OneBeacon Runoff Discontinued Operations | Dec. 31, 2013 OneBeacon Runoff Discontinued Operations Rating | Dec. 31, 2012 OneBeacon Runoff Discontinued Operations | Dec. 31, 2011 OneBeacon Runoff Discontinued Operations | Mar. 31, 2013 OneBeacon Runoff Discontinued Operations | Dec. 31, 2012 Minimum OneBeacon Runoff Discontinued Operations | Mar. 31, 2013 Minimum OneBeacon Runoff Discontinued Operations | Dec. 31, 2012 Maximum OneBeacon Runoff Discontinued Operations | Mar. 31, 2013 Maximum OneBeacon Runoff Discontinued Operations | Mar. 31, 2013 Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense, Cause [Domain] OneBeacon Runoff Discontinued Operations | Dec. 31, 2013 N I C O Cover [Member] OneBeacon Runoff | Dec. 31, 2013 G R C Cover [Member] OneBeacon Runoff | May 31, 2001 General Reinsurance Corporation G R C Cover [Member] OneBeacon Runoff | Dec. 31, 2013 Workers compensation OneBeacon Runoff Discontinued Operations | Dec. 31, 2013 Personal Auto Liability [Member] OneBeacon Runoff Discontinued Operations | Dec. 31, 2013 Property and Casualty, Personal Insurance Product Line [Member] OneBeacon Runoff Discontinued Operations | Dec. 31, 2012 Workers compensation loss reserves [Member] OneBeacon Runoff Discontinued Operations | Dec. 31, 2013 Insurance Claims [Member] OneBeacon Runoff Discontinued Operations | Dec. 31, 2013 Change in Assumption of Liability [Member] OneBeacon Runoff Discontinued Operations | Jun. 30, 2013 Safeco v. American International Group, Inc. [Member] OneBeacon Runoff Discontinued Operations
Reinsurance Retention Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax |  |  |  |  |  |  |  |  | $ 46.6 | $ (91.0) | $ 658.3 |  | $ 46.6 | $ (91.0) | $ 658.3 | $ 677.5 | $ 0.5 | $ (19.2) |  |  |  |  |  | $ 44.9 | $ 91.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of insurance companies sold |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and loss adjustment expense reserves | 3,079.3 |  |  |  | 3,168.9 |  |  |  | 3,079.3 | 3,168.9 | 5,702.3 | 5,736.8 | 1,793.1 | 2,052.6 |  |  |  |  |  |  |  | 188.4 |  | 188.4 |  |  | 166.0 |  | 215.0 |  | 668.0 | 78.9 |  |  |  |  |  |  |  |  |  |
Net income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | 4.5 | (115.0) | 621.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and LAE incurred relating prior year losses |  |  |  |  |  |  |  |  | 48.4 | 41.9 | 76.7 |  |  |  |  |  |  |  |  |  |  | 71.5 |  | (6.5) | (40.0) | (26.7) |  |  |  |  |  |  |  |  |  |  | (16.9) | (78.9) |  | (5.4) | (12.6) |
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 36.6 |  |  |  |  |  |
Loss and loss adjustment expenses |  |  |  |  |  |  |  |  | 1,040.5 | 1,193.9 | 1,174.3 |  | 78.9 | 48.4 | 574.9 |  |  |  |  |  |  |  | 7.4 | 78.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other revenue |  |  |  |  |  |  |  |  | 57.5 | 100.3 | (10.0) |  | 10.8 | 0 | 55.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.8
liability for unpaid claims and claims adjustment expenses, incurred claims, prior years, after-tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 46.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net loss from discontinued operations, net of tax | (0.3) | 0.4 | 3.9 | 0.5 | 0.5 | (106.8) | 0.5 | (9.2) | (42.1) | (24.0) | (36.7) |  | (42.1) | (24.0) | (36.7) |  |  |  |  |  |  | 46.6 |  |  | 24.0 |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.0 |  |  |
Maximum Number of Days to Extend the Termination Date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 90 days |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 69.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of contracts seller caused One Beacon to purchase prior to acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverables, gross | 453.5 |  |  |  | 447.0 |  |  |  | 453.5 | 447.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,500.0 | 570.0 |  |  |  |  |  |  |  |
Workers compensation discount percent |  |  |  |  |  |  |  |  | 3.50% | 3.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.50% |  | 4.50% |  |  |  |  |  |  |  |  |  |  |  |
Workers compensation discount amount | 3.0 |  |  |  | 4.6 |  |  |  | 3.0 | 4.6 |  |  |  |  |  |  |  |  |  | 61.7 | 77.9 |  |  |  | 15.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adverse loss reserve development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400.0 |  |  |  |  |  |  |
Loss reserves ceded at acquisition |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 170.0 |  |  |  |  |  |  |
Reinsurance recoverable on paid and unpaid losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,243.7 |  | 1,243.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
AM Best Rating System Number Of Ratings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 16 |  | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses | 25.4 |  |  |  | 17.9 |  |  |  | 25.4 | 17.9 |  |  |  | 15.6 |  |  |  |  |  |  |  | 10.7 |  | 10.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverables |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,741.6 |  | 1,741.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase accounting adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 136.9 |  | $ 136.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Amount of income (loss) from a disposal group, net of income tax before extraordinary items allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e957-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.12) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 13 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.14) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 15 -Article 5 + Details
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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + 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 (a),(b),(c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Represents the number of reinsurance contracts the seller caused subsidiary entity to purchase from subsidiaries of Berkshire Hathaway Inc. immediately prior to the entity's acquisition of subsidiary entity. + References No definition available. + Details
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