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Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |  | 1 Months Ended | 12 Months Ended | 3 Months Ended | 12 Months Ended |  |  | 9 Months Ended | 12 Months Ended |  |  | 12 Months Ended |  |  | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 3 Months Ended | 1 Months Ended | 3 Months Ended | 12 Months Ended |
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2013 2012 OBH Senior Notes | Dec. 31, 2012 2012 OBH Senior Notes | Mar. 31, 2007 SIG Senior Notes | Dec. 31, 2013 SIG Senior Notes | Dec. 31, 2012 SIG Senior Notes | Dec. 31, 2011 SIG Senior Notes | Dec. 31, 2013 WTM Bank Facility | Dec. 31, 2013 WTM Bank Facility | Dec. 31, 2012 WTM Bank Facility | Aug. 12, 2011 WTM Bank Facility | Sep. 30, 2013 Prior W T M Bank Facility | Dec. 31, 2013 Prior W T M Bank Facility | Dec. 31, 2012 Prior W T M Bank Facility | Dec. 31, 2011 Prior W T M Bank Facility | Jan. 31, 2013 Prior W T M Bank Facility | Aug. 12, 2011 Prior W T M Bank Facility | Dec. 31, 2011 Old Lyme | Dec. 31, 2013 Old Lyme | Dec. 31, 2012 Old Lyme | Nov. 30, 2012 One Beacon US Holdings Inc. 2012 OBH Senior Notes | Dec. 31, 2013 One Beacon US Holdings Inc. 2012 OBH Senior Notes | May 31, 2003 One Beacon US Holdings Inc. 2003 OBH Senior Notes | Dec. 31, 2012 One Beacon US Holdings Inc. 2003 OBH Senior Notes | Dec. 31, 2011 One Beacon US Holdings Inc. 2003 OBH Senior Notes | Apr. 30, 2011 One Beacon US Holdings Inc. Senior Notes 2003 OBH Senior Notes | Dec. 31, 2012 One Beacon US Holdings Inc. Senior Notes 2003 OBH Senior Notes | Sep. 30, 2010 Sirius Internation Group Ltd Sierra Note | Mar. 31, 2004 Sirius Internation Group Ltd Sierra Note | Dec. 31, 2011 Sirius Internation Group Ltd Sierra Note | Dec. 31, 2011 Sirius Internation Group Ltd Sierra Note | Dec. 31, 2008 Sirius Internation Group Ltd Sierra Note | Dec. 31, 2007 Sirius Internation Group Ltd Sierra Note | Dec. 31, 2005 Sirius Internation Group Ltd Sierra Note | Sep. 15, 2010 Sirius Internation Group Ltd Sierra Note
Debt Instrument |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument at face value |  |  |  | $ 275.0 | $ 275.0 | $ 400.0 | $ 400.0 | $ 400.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 275.0 |  | $ 700.0 |  |  |  |  |  | $ 62.0 |  |  |  |  |  |
Debt | 676.4 | 751.2 |  | 274.7 | 274.7 |  | 399.6 | 399.4 |  | 0 | 0 | 0 |  |  | 0 | 75.0 |  | 75.0 |  | 2.1 | 2.1 | 2.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value at which debt was issued |  |  |  |  |  | 99.715% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 99.90% |  | 99.70% |  |  |  |  |  |  |  |  |  |  |  |
Issuance of debt, net of debt issuance costs | 0 | 271.9 | 0 |  |  | 392.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 272.9 |  | 693.4 |  |  |  |  |  |  |  |  |  |  |  |
Debt issue costs |  |  |  |  |  | 3.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.8 |  | 7.3 |  |  |  |  |  |  |  |  |  |  |  |
Underwriting discount |  |  |  |  |  | 2.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.8 |  | 4.5 |  |  |  |  |  |  |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  |  | 6.375% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.60% |  | 5.875% |  |  |  |  |  | 4.00% |  |  |  |  |  |
Effective yield (as a percent) |  |  |  |  |  | 6.50% |  |  |  | 3.65% | 3.65% |  |  |  | 2.83% | 3.53% |  |  |  |  |  |  | 4.70% |  | 6.00% |  |  |  |  |  |  |  |  |  |  |  |
Interest expense on debt | 42.5 | 44.8 | 55.2 |  |  |  | 26.2 | 26.2 | 26.2 |  | 0.3 |  |  |  | 2.9 | 1.7 | 3.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.5 |  |  |  |  |
Debt Instrument, Repurchase Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 269.8 |  |  |  |  |  |  |  |
Guarantee fee basis points per annum on outstanding principal amount of debt (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.50% |  |  |  |  |  |  |  |  |  |  |  |
Expenses related to issuance, including underwriting fees |  |  |  |  |  | 3.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.8 |  | 7.3 |  |  |  |  |  |  |  |  |  |  |  |
Repayments of lines of credit |  |  |  |  |  |  |  |  |  | 50.0 |  |  |  | 150.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount of senior notes purchased and retired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 150.0 |  |  |  |  |  |  |  |  |
Aggregate purchase price of senior notes purchased and retired | 0 | 275.9 | 161.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 161.6 | 275.9 |  |  |  |  |  |  |  |
Loss on retirement of debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (12.0) | (6.3) |  |  |  |  |  |  |  |
Interest Expense |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13.0 |  | 16.9 | 20.5 |  |  |  |  |  |  |  |  |  |
Write off of deferred issuance costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.2 |  |  |  |  |  |  |  |
Loss from interest rate hedge recorded in other comprehensive income |  |  |  |  |  | (2.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unamortized loss remaining in accumulated other comprehensive income |  |  |  |  |  |  | 1.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |  | 425.0 |  |  |  |  |  | 375.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Draw down of revolving line of credit | 200.0 | 150.0 | 0 |  |  |  |  |  |  | 50.0 |  |  |  | 150.0 |  | 150.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of principal amount of debt subject to upward or downward adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of purchase note |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument Term (in years) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  | 6 years |  |  |  |  |  |
Amount of debt available for adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 58.0 |  |  |  |  |  |
Interest Paid | 42.6 | 44.6 | 59.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.8 |  |  |  |  |  |  |
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
Repayments of debt, principal | 275.0 | 75.0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  | 75.0 |  |  |  |  |  |  |  |  |  |  |  |  |  | 33.0 |  |  |  |  |  |  |
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 |  |  |  |  |  |  |

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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 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 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 Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: 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) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585 + Details
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- Definition The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt. + References No definition available. + Details
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- Definition Amount of cash inflow from contractual arrangement with the lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(f)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph f -Article 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
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- Definition The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations. + References No definition available. + Details
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- Definition Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(f)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph f -Article 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 + Details
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- Definition Write-off of amounts previously capitalized as debt issuance cost in an extinguishment of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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- Definition Represents the amount of debt available for adjustment to reflect favorable or adverse loss reserve development on the acquired reserve portfolio and run-off of remaining policies in force. + References No definition available. + Details
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- Definition Represents the price of debt issued as a percentage of its par value. + References No definition available. + Details
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- Definition Represents the percentage of principal amount, if debt instrument which is subject to upward adjustments for favorable loss reserve development and downward adjustments for any adverse loss reserve development. + References No definition available. + Details
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- Definition Represents the amount of capitalized underwriting costs associated with the issuance of debt instruments that will be charged against earnings over the life of the debt instruments to which such costs pertain. Such amount is before the consideration of accumulated amortization. + References No definition available. + Details
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- Definition Represents guarantee fee which is equal to basis points per annum on the outstanding principal amount, paid to guarantor for unconditionally guarantying the payment of principal and interest of the debt. + References No definition available. + Details
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- Definition This element represents the amount for the accrued interest portion of the debt repayments reimbursed. + References No definition available. + Details
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- Definition This element represents the amount of debt and accrued interest repayments reimbursed. + References No definition available. + Details
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- Definition This element represents the amount for the principal portion of the debt repayments reimbursed. + References No definition available. + Details
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- Definition Cash outflow for the repayment of borrowings, including principal and accrued interest. + References No definition available. + Details
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