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Income Taxes (Net Operating and Capital Loss Carryforwards) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  | 3 Months Ended | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2012 UNITED STATES | Dec. 31, 2012 Luxembourg | Dec. 31, 2012 SWEDEN | Dec. 31, 2012 Domestic Tax Authority | Dec. 31, 2012 Sirius Re Holdings Inc. | Dec. 31, 2012 Limited Liability Company | Dec. 31, 2011 Limited Liability Company | Dec. 31, 2010 Limited Liability Company
Summary of net operating loss and capital loss carryforwards, the expiration dates and the deferred tax assets thereon |  |  |  |  |  |  |  |  |  |  |
Fees from Revenue Sharing Agreement with Related Party |  |  |  |  |  |  |  |  | $ 0.5 | $ 0.2 | $ 0.7
2013 | 0 |  |  |  |  |  |  |  |  |  |
2014-2018 | 0 |  |  | 0 | 0 | 0 |  |  |  |  |
2019-2023 | 20.2 |  |  | 20.2 | 0 | 0 |  |  |  |  |
2024-2033 | 467.0 |  |  | 467.0 | 0 | 0 |  |  |  |  |
No expiration date | 1,743.7 |  |  | 0 | 1,743.6 | 0.1 |  |  |  |  |
Total | 2,230.9 |  |  | 487.2 | 1,743.6 | 0.1 |  |  |  |  |
Net deferred tax assets |  |  |  |  |  |  |  |  |  |  |
Gross deferred tax asset | 674.7 |  |  | 165.2 | 509.5 | 0 |  |  |  |  |
Valuation allowance | (224.3) |  |  | (32.3) | (192.0) | 0 |  |  |  |  |
Net deferred tax asset | 450.4 |  |  | 132.9 | 317.5 | 0 |  |  |  |  |
Net operating loss carryforwards expected to be utilized |  |  |  |  | 1,308.5 |  |  |  |  |  |
Net operating loss carryforwards, subject to an annual limitation |  |  |  |  |  |  | 29.0 |  |  |  |
Foreign tax credit carryforwards, subject to expiration |  |  |  |  |  |  | 10.6 | 4.6 |  |  |
Valuation allowance | 14.1 | (128.2) | (2.6) |  |  |  |  | 3.9 |  |  |
Low-income housing tax credit | 4.8 |  |  |  |  |  |  |  |  |  |
Tax credit carryforwards, investments |  |  |  |  |  |  | 0.2 |  |  |  |
Alternative minimum tax credit carryforwards, not subject to expiration | 1.5 |  |  |  |  |  |  |  |  |  |
Deferred tax benefit due to increase in its NOLs |  |  | $ 11.9 |  |  |  |  |  |  |  |

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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible foreign tax credit carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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- Definition The portion of the difference between total income tax expense or benefit as reported in the Income Statement and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations attributable to changes in the valuation allowance for deferred tax assets in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 109 -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=21917399&loc=d3e32123-109318 + Details
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