Contingencies	6 Months Ended
Jun. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies Legal Contingencies White Mountains is 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 are directly related to, claims activity. White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed 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 White Mountains’s financial condition, results of operations or cash flows. The following summarizes White Mountains’s significant legal contingencies, ongoing non-claims related litigation or arbitration as of June 30, 2017: Litigation Related to the OneBeacon Transaction On June 2, 2017, Stephen Bushansky, a purported OneBeacon shareholder, filed a class action complaint against OneBeacon and each of OneBeacon’s directors in the U.S. District Court for the District of Minnesota (the “Minnesota Court”), purportedly on behalf of OneBeacon’s public shareholders. Thereafter, three additional lawsuits were filed in the Minnesota Court by additional purported shareholders, Darrin Dickers, Raymond Martino and Robert Berg (collectively with Bushansky, the “Plaintiffs”). The complaints in each pending class action allege that OneBeacon’s preliminary proxy statement filed with the U.S. Securities and Exchange Commission (“SEC”) omitted or misrepresented certain material information, allegedly in violation of Sections 14(a) and 20(a) of the Securities Exchange Act of 1934 and SEC Rule 14a-9 and sought to enjoin OneBeacon and Intact Financial Corporation from closing the OneBeacon Transaction, or if the transaction closes, to award Plaintiffs damages and costs. On June 26, 2017, Plaintiffs jointly filed a motion for preliminary injunction to enjoin the shareholder vote on the OneBeacon Transaction, but withdrew their motion on July 7, 2017. OneBeacon’s shareholders approved the OneBeacon Transaction on July 18, 2017 at a special general meeting of shareholders. Plaintiffs have not dismissed their cases and OneBeacon’s responsive pleadings are due August 14, 2017. OneBeacon believes the cases lack merit and continues to vigorously defend this litigation. Tribune Company 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 U.S. District Court for the Southern District of New York (the “SDNY”). 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 and OneBeacon-sponsored benefit plans received approximately $32.0 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’ appealed. On March 29, 2016, a three judge panel of the U.S Second Circuit Court of Appeals affirmed the dismissal of the Noteholder Actions. On July 22, 2016, the Plaintiff’s petition to the Second Circuit for reconsideration or for a rehearing en banc was denied in full. On September 9, 2016 the Plaintiffs filed for a writ of certiorari, seeking review in the U. S. Supreme Court. In addition, OneBeacon and OneBeacon-sponsored benefit plans 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 SDNY and was stayed pending the motion to dismiss in the Noteholder Actions. An omnibus motion to dismiss the shareholder defendants in the Committee Action was filed in May 2014 and the motion was granted on January 6, 2017. The plaintiff has requested permission to move the SDNY to certify the decision as a final judgment capable of immediate appeal. No amount has been accrued in connection with this matter as of June 30, 2017, as the amount of loss, if any, cannot be reasonably estimated.

Contingencies

Legal Contingencies

White Mountains is 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 are directly related to, claims activity.

White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed 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 White Mountains’s financial condition, results of operations or cash flows.

The following summarizes White Mountains’s significant legal contingencies, ongoing non-claims related litigation or arbitration as of June 30, 2017:

Litigation Related to the OneBeacon Transaction

On June 2, 2017, Stephen Bushansky, a purported OneBeacon shareholder, filed a class action complaint against OneBeacon and each of OneBeacon’s directors in the U.S. District Court for the District of Minnesota (the “Minnesota Court”), purportedly on behalf of OneBeacon’s public shareholders. Thereafter, three additional lawsuits were filed in the Minnesota Court by additional purported shareholders, Darrin Dickers, Raymond Martino and Robert Berg (collectively with Bushansky, the “Plaintiffs”). The complaints in each pending class action allege that OneBeacon’s preliminary proxy statement filed with the U.S. Securities and Exchange Commission (“SEC”) omitted or misrepresented certain material information, allegedly in violation of Sections 14(a) and 20(a) of the Securities Exchange Act of 1934 and SEC Rule 14a-9 and sought to enjoin OneBeacon and Intact Financial Corporation from closing the OneBeacon Transaction, or if the transaction closes, to award Plaintiffs damages and costs.

On June 26, 2017, Plaintiffs jointly filed a motion for preliminary injunction to enjoin the shareholder vote on the OneBeacon Transaction, but withdrew their motion on July 7, 2017. OneBeacon’s shareholders approved the OneBeacon Transaction on July 18, 2017 at a special general meeting of shareholders. Plaintiffs have not dismissed their cases and OneBeacon’s responsive pleadings are due August 14, 2017. OneBeacon believes the cases lack merit and continues to vigorously defend this litigation.

Tribune Company

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 U.S. District Court for the Southern District of New York (the “SDNY”).

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 and OneBeacon-sponsored benefit plans received approximately $32.0 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’ appealed. On March 29, 2016, a three judge panel of the U.S Second Circuit Court of Appeals affirmed the dismissal of the Noteholder Actions. On July 22, 2016, the Plaintiff’s petition to the Second Circuit for reconsideration or for a rehearing en banc was denied in full. On September 9, 2016 the Plaintiffs filed for a writ of certiorari, seeking review in the U. S. Supreme Court.

In addition, OneBeacon and OneBeacon-sponsored benefit plans 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 SDNY and was stayed pending the motion to dismiss in the Noteholder Actions. An omnibus motion to dismiss the shareholder defendants in the Committee Action was filed in May 2014 and the motion was granted on January 6, 2017. The plaintiff has requested permission to move the SDNY to certify the decision as a final judgment capable of immediate appeal. No amount has been accrued in connection with this matter as of June 30, 2017, as the amount of loss, if any, cannot be reasonably estimated.