Third Party Reinsurance	3 Months Ended
Third Party Reinsurance	Mar. 31, 2016
Reinsurance Disclosures [Abstract]
Third Party Reinsurance	Third Party Reinsurance In the normal course of business, White Mountains’s insurance subsidiaries may seek to limit losses that may arise from catastrophes or other events by reinsuring with third party reinsurers. White Mountains remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts. OneBeacon At March 31, 2016, OneBeacon had $16.6 million and $150.4 million of reinsurance recoverables on paid and unpaid losses. At December 31, 2015, OneBeacon had $7.5 million and $186.0 million of reinsurance recoverables on paid and unpaid losses. Reinsurance contracts do not relieve OneBeacon of its obligation to its policyholders. OneBeacon is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. OneBeacon monitors the financial strength and ratings of its reinsurers on an ongoing basis. Uncollectible amounts related to the ongoing specialty business historically have not been significant. In the first quarter of 2016, OneBeacon entered into a 50% quota share reinsurance agreement related to its Financial Institutions business.

The entire disclosure pertaining to the existence, magnitude and information about insurance that has been ceded to or assumed from another insurance company, including the methodologies and assumptions used in determining recorded amounts.