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Retirement and Postretirement Plans (Qualifed, Non-qualified, and Accumulated Benefit Obligations) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011
Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets |  |  |  |
Projected benefit obligation | $ 27.6 |  | $ 26.5 |
Accumulated benefit obligation | 27.6 |  | 26.5 |
Fair value of plan assets | 0 |  | 0 |
Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets |  |  |  |
Projected benefit obligation | 91.9 |  | 86.5 |
Accumulated benefit obligation | 91.9 |  | 86.5 |
Fair value of plan net assets | 124.7 | [1] | 120.8 | [1]
Amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis noncontrolling interest |  |  |  |
Accumulated other comprehensive (loss) income beginning balance | (16.7) |  | 0.5 |
Increase (decrease) in accumulated other comprehensive income (loss): |  |  |  |
Amortization of net actuarial losses recognized during the year | 0.8 |  | 0.5 |
Net actuarial (losses) gains occurring during the year | (5.9) | [2] | (18.2) | [2]
Other adjustments | 0.6 |  | 0.5 |
Accumulated other comprehensive loss ending balance | (21.2) |  | (16.7) |
OneBeacon Ltd. |  |  |  |
Increase (decrease) in accumulated other comprehensive income (loss): |  |  |  |
Amount expected to be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost during 2012 | $ 0.9 |  |  |
[1] | Includes receivables related to securities sold, interest and dividends as well as payables related to securities purchased.
[2] | Net actuarial losses resulted from a decrease in investment returns on plan assets in the year ended December 31, 2011 and changes in assumptions in estimating the projected benefit obligation in the years ended December 31, 2012 and 2011

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- Definition The pretax total of net gain (loss), prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph i -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 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=20435746&loc=SL7669646-108580 + Details
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- Definition The aggregate amount in accumulated other comprehensive income expected to be recognized as a component of net periodic benefit cost over the fiscal year that follows the most recent annual statement of financial position presented, including the net gain (loss), net prior service cost or credit, and net transition asset or obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (s) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 158 -Paragraph 7 -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph s -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 If aggregate disclosures are presented, the aggregate fair value of plan assets for pension plans with accumulated benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=21916443&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e2417-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 6 -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. + Details
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