Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 383.3	$ 343.1
OneBeacon \| 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	274.8	274.7
OneBeacon \| Book Value \| 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	274.8	274.7
OneBeacon \| Estimate of Fair Value Measurement [Member] \| 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	279.8	286.0
Tranzact [Member] \| Tranzact Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term Line of Credit	95.8	68.7
Tranzact [Member] \| Book Value \| Tranzact Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term Line of Credit	93.8	67.4
Tranzact [Member] \| Estimate of Fair Value Measurement [Member] \| Tranzact Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term Line of Credit	$ 94.6	$ 68.7

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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