Significant Transactions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Significant Transactions
Gain on sale of subsidiary - Essentia		$ 23.0	$ 0
Excess of FV of aquired net assets over cost - American Fuji		6.9
OneBeacon \| Essentia
Significant Transactions
Gain on sale of subsidiary - Essentia		23.0
Gain on sale of Essentia, after tax		15.0
Premiums written	$ 58.0		$ 93.3
Premiums written, percentage	19.80%		15.70%

Net of tax amount of gain (loss) classified as extraordinary item, including amount attributable to noncontrolling interests.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

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-Paragraph 38

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Name Accounting Principles Board Opinion (APB)

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

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Reference 11: http://www.xbrl.org/2003/role/presentationRef

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Reference 12: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 15

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+ Details