Legal Contingencies	3 Months Ended
Mar. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Legal Contingencies	Legal Contingencies OneBeacon, and the insurance and reinsurance industry in general, is routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or directly relate to, claims activity. OneBeacon's estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 3—"Unpaid Loss and Loss Adjustment Expense (LAE) Reserves." OneBeacon evaluates its exposure to non-claims related litigation and arbitration and establishes accruals for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. Disclosure of litigation and arbitration is made if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on OneBeacon's financial condition, full year results of operations, or cash flows. The following summarizes significant ongoing non-claims related litigation or arbitration as of March 31, 2014: Deutsche Bank Litigation In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune's leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the United States District Court for the Southern District of New York. Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon has entered into a joint defense agreement with other affiliates of White Mountains that are defendants in the action. OneBeacon and OneBeacon-sponsored benefit plans received approximately $32 million for Tribune common stock tendered in connection with the LBO. The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs have filed a notice of appeal. In addition, OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune, along with thousands of former Tribune shareholders, have been named as defendants in an adversary proceeding brought by the Official Committee of Unsecured Creditors of the Tribune Company (the “Committee”), on behalf of the Tribune Company, which seeks to avoid the repurchase of shares by Tribune in the LBO on a theory of intentional fraudulent transfer (the “Committee Action”). Tribune emerged from bankruptcy in 2012, and a litigation trustee replaced the Committee as plaintiff in the Committee Action. This matter was consolidated for pretrial matters with the Noteholder Actions in the United States District Court for the Southern District of New York and was stayed pending the motion to dismiss in the Noteholder Action. The Committee Action will proceed upon the lifting of the stay and a scheduling order.

Legal Contingencies

OneBeacon, and the insurance and reinsurance industry in general, is routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or directly relate to, claims activity. OneBeacon's estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 3—"Unpaid Loss and Loss Adjustment Expense (LAE) Reserves."

OneBeacon evaluates its exposure to non-claims related litigation and arbitration and establishes accruals for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. Disclosure of litigation and arbitration is made if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on OneBeacon's financial condition, full year results of operations, or cash flows.

The following summarizes significant ongoing non-claims related litigation or arbitration as of March 31, 2014:

Deutsche Bank Litigation

In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune's leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the United States District Court for the Southern District of New York. Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon has entered into a joint defense agreement with other affiliates of White Mountains that are defendants in the action. OneBeacon and OneBeacon-sponsored benefit plans received approximately $32 million for Tribune common stock tendered in connection with the LBO.

The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs have filed a notice of appeal.

In addition, OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune, along with thousands of former Tribune shareholders, have been named as defendants in an adversary proceeding brought by the Official Committee of Unsecured Creditors of the Tribune Company (the “Committee”), on behalf of the Tribune Company, which seeks to avoid the repurchase of shares by Tribune in the LBO on a theory of intentional fraudulent transfer (the “Committee Action”). Tribune emerged from bankruptcy in 2012, and a litigation trustee replaced the Committee as plaintiff in the Committee Action. This matter was consolidated for pretrial matters with the Noteholder Actions in the United States District Court for the Southern District of New York and was stayed pending the motion to dismiss in the Noteholder Action. The Committee Action will proceed upon the lifting of the stay and a scheduling order.