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Debt (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  | 1 Months Ended | 12 Months Ended |  | 1 Months Ended | 12 Months Ended |  | 1 Months Ended | 12 Months Ended |  | 1 Months Ended | 12 Months Ended | 3 Months Ended | 1 Months Ended | 12 Months Ended |  | 12 Months Ended |  |  |
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2011 OneBeacon Ltd. | May 31, 2003 OBH Senior Notes | Dec. 31, 2011 OBH Senior Notes | Dec. 31, 2010 OBH Senior Notes | Dec. 31, 2009 OBH Senior Notes | May 30, 2003 OBH Senior Notes | Apr. 30, 2011 OBH Senior Notes Tender offer | Jun. 30, 2010 OBH Senior Notes Tender offer | Dec. 31, 2010 OBH Senior Notes Tender offer | Dec. 31, 2010 OBH Senior Notes Repurchase and retirement of debt | Dec. 31, 2011 OBH Senior Notes OneBeacon Ltd. | Mar. 31, 2007 SIG Senior Notes | Dec. 31, 2011 SIG Senior Notes | Dec. 31, 2010 SIG Senior Notes | Dec. 31, 2009 SIG Senior Notes | Aug. 12, 2011 Prior WTM Bank Facility | Dec. 31, 2005 Mortgage Note OneBeacon Ltd. Y | Dec. 31, 2009 Mortgage Note OneBeacon Ltd. | Dec. 31, 2009 Mortgage Note OneBeacon Ltd. Interest rate swap | Dec. 31, 2011 Sierra Note | Sep. 30, 2010 Sierra Note Sirius Group | Mar. 31, 2004 Sierra Note Sirius Group Y | Dec. 31, 2010 Sierra Note Sirius Group | Dec. 31, 2008 Sierra Note Sirius Group | Dec. 31, 2007 Sierra Note Sirius Group | Dec. 31, 2005 Sierra Note Sirius Group | Sep. 15, 2010 Sierra Note Sirius Group | Dec. 31, 2010 Atlantic Specialty Note OneBeacon Ltd. | Mar. 31, 2004 Atlantic Specialty Note OneBeacon Ltd. Y | Dec. 30, 2011 Old Lyme Y | Aug. 12, 2011 WTM Bank Facility
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument at face value |  |  |  |  | $ 700.0 | $ 269.9 | $ 419.9 |  |  |  |  |  |  |  | $ 400.0 | $ 400.0 | $ 400.0 |  |  |  |  |  |  |  | $ 62.0 |  |  |  |  |  |  |  |  |
Unamortized original issue discount |  |  |  |  |  | (0.1) | (0.3) |  |  |  |  |  |  |  |  | (0.7) | (0.8) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total debt | 671.2 | 818.8 |  |  |  | 269.8 | 419.6 |  |  |  |  |  |  |  |  | 399.3 | 399.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.1 |
Percentage of par value at which debt was issued |  |  |  |  | 99.70% |  |  |  |  |  |  |  |  |  | 99.715% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash received from debt issuance |  |  |  |  | 693.4 |  |  |  |  |  |  |  |  |  | 392.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt issue costs |  |  |  |  |  |  |  |  | 7.3 |  |  |  |  |  | 3.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Underwriting discount |  |  |  |  | 4.5 |  |  |  |  |  |  |  |  |  | 2.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate ( as a percent) |  |  |  |  |  | 5.875% |  |  |  |  |  |  |  |  |  | 6.375% |  |  |  |  |  |  |  |  | 4.00% |  |  |  |  |  |  | 5.20% |  |
Effective yield (as a percent) |  |  |  |  |  | 6.00% |  |  |  |  |  |  |  |  |  | 6.49% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantee fee basis points per annum on outstanding principal amount of debt (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.25% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum voting interest in subsidiary (as a percent) |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum guarantee fee (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.25% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expenses related to issuance, including underwriting fees |  |  |  |  |  |  |  |  | 7.3 |  |  |  |  |  | 3.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest expense, inclusive of amortization of issuance costs | 55.2 | 57.3 | 70.8 |  |  | 20.5 | 29.5 | 38.1 |  |  |  |  |  |  |  | 26.1 | 26.1 | 26.1 |  |  |  |  | 5.5 |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount of senior notes purchased and retired |  |  |  |  |  |  |  |  |  | 150.0 | 156.4 | 29.7 | 1.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate purchase price of senior notes purchased and retired | 161.6 | 197.3 | 63.1 |  |  |  |  |  |  | 161.6 | 165.4 | 30.8 | 1.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on retirement of debt |  |  |  |  |  |  |  |  |  | 12.0 | 9.6 | 1.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss from interest rate hedge recorded in other comprehensive income |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unamortized loss remaining in accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 475.0 |  |  |  |  |  |  |  |  |  |  |  |  | 20.0 |  | 375.0
Percentage of principal amount of debt subject to upward or downward adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |
Principal amount of purchase note |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6 |
Mortgage note outstanding |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument Term (in years) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 18 |  |  |  |  | 6 |  |  |  |  |  |  | 10 | 5 |
Variable interest rate basis |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30-day LIBOR |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount paid to settle interest rate swap |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.4 |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) recorded on settlement of derivative, pre tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.4 |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) recorded on settlement of derivative, after tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.8 |  |  |  |  |  |  |  |  |  |  |  |
After-tax change in the fair value of the interest rate swap | 92.9 | 127.0 | (158.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.0 |  |  |  |  |  |  |  |  |  |  |  |
Amount of debt available for adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 58.0 |  |  |  |  |  |  |  |  |
Interest paid | 59.0 | 64.1 | 69.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase decrease in the principal of the note issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (29.0) | 1.9 | (5.2) | 9.1 | (22.8) |  |  |  |  |
Undisputed amounts, principal and interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 42.8 |  |  |  |
Repayment of debt, principal |  | 14.0 | 242.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33.0 |  |  |  |  |  |  | 14.0 |  |  |
Repayment of debt, accrued interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.8 |  |  |  |  |  |  |  |  |  |
Reimbursement of debt repayment by Berkshire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 36.7 |  |  |  |  |  |  |  |  |  |
Reimbursement of debt repayment by Berkshire, principal portion |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33.0 |  |  |  |  |  |  |  |  |  |
Reimbursement of debt prepayment by Berkshire, accrued interest portion |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.7 |  |  |  |  |  |  |  |  |  |
Interest rate if outstanding principal amount of debt exceeds specified limit (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.60% |  |
Specified limit of outstanding principal amount of debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15.0 |  |
Contractual repayments of debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Due in two to three years | 269.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Due in four to five years | 2.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Due after five years | 400.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | $ 672.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6920043&loc=d3e653-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6887037&loc=d3e80784-113994 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 130 -Paragraph 26 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6920043&loc=d3e637-108580 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=6920043&loc=d3e681-108580 + Details
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- Definition Amount of cash paid to acquire the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 51 -Subparagraph d -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141R -Paragraph 68 -Subparagraph f(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. + References No definition available. + Details
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- Definition The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 21 -Paragraph 16, 20 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400 + Details
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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number APB14-1 -Paragraph 31 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 21 -Paragraph 16, 20 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400 + Details
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- Definition The effective portion of losses on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 205G -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44C -Subparagraph b -Clause 2 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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