Nature of Operations and Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2014
Accounting Policies [Abstract]
Recently Adopted Changes in Accounting Principles	Recently Adopted Changes in Accounting Principles Unrecognized Tax Benefits Effective January 1, 2014, OneBeacon adopted ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (ASC 740). The new ASU requires balance sheet presentation of an unrecognized tax benefit as a reduction of a deferred tax asset for a net operating loss ("NOL") carryforward or tax credit carryforward rather than as a liability. The exception is in circumstances where a carryforward is not available to settle the additional taxes that might arise upon disallowance of the tax position under the tax law of the applicable jurisdiction. Prior to the issuance of ASU 2013-11, the guidance for unrecognized tax benefits under ASC 740 did not provide explicit guidance on whether an entity should present an unrecognized tax benefit as a liability or as a reduction of NOL carryforwards or other tax credits. In circumstances where an NOL carryforward is not available to offset settlement of any additional taxes arising from a disallowed tax position, the unrecognized tax benefit should be presented as a liability. The new guidance becomes effective for fiscal periods beginning on or after December 15, 2013 and should be applied prospectively to all unrecognized tax benefits that exist at the effective date. Retrospective adoption is allowed. This adoption did not have any impact on the Company's financial statements. Recently Issued Accounting Pronouncements Qualified Affordable Housing Projects In January 2014 the FASB issued ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects ("QAHP"), which permits companies to make an accounting policy election to account for investments in a QAHP using the proportional amortization method, if certain conditions are met. Under this method, the initial cost of the investment is amortized in proportion to the tax credits and other tax benefits received, with the net investment performance recognized in the income statement as a component of income tax expense. The new guidance also requires certain new disclosures for all QAHP investments. ASU 2014-01 is effective for annual and interim reporting periods beginning after December 15, 2014 and must be applied retrospectively to all periods presented upon adoption. The Company currently holds an investment in a QAHP that is accounted for under the equity method and does not expect the adoption to have a material impact on its financial position, results of operations or cash flows. Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity In April 2014, the FASB issued ASU 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity to reduce diversity in practice for reporting discontinued operations. Under the previous guidance, any component of an entity that was a reportable segment, an operating segment, a reporting unit, a subsidiary, or an asset group was eligible for discontinued operations presentation. The revised guidance only allows disposals of components of an entity that represent a strategic shift (e.g., disposal of a major geographical area, a major line of business, a major equity method investment, or other major parts of an entity) and that have a major effect on a reporting entity’s operations and financial results to be reported as discontinued operations. The revised guidance also requires expanded disclosure in the financial statements for discontinued operations as well as for disposals of significant components of an entity that do not qualify for discontinued operations presentation. The updated guidance is effective for periods beginning after December 15, 2014. The Company currently has operations that are reported as discontinued operations and does not expect the adoption of this guidance to have a material effect on its financial position, results of operations, or cash flows.

Recently Adopted Changes in Accounting Principles

Unrecognized Tax Benefits

Effective January 1, 2014, OneBeacon adopted ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (ASC 740). The new ASU requires balance sheet presentation of an unrecognized tax benefit as a reduction of a deferred tax asset for a net operating loss ("NOL") carryforward or tax credit carryforward rather than as a liability. The exception is in circumstances where a carryforward is not available to settle the additional taxes that might arise upon disallowance of the tax position under the tax law of the applicable jurisdiction. Prior to the issuance of ASU 2013-11, the guidance for unrecognized tax benefits under ASC 740 did not provide explicit guidance on whether an entity should present an unrecognized tax benefit as a liability or as a reduction of NOL carryforwards or other tax credits. In circumstances where an NOL carryforward is not available to offset settlement of any additional taxes arising from a disallowed tax position, the unrecognized tax benefit should be presented as a liability. The new guidance becomes effective for fiscal periods beginning on or after December 15, 2013 and should be applied prospectively to all unrecognized tax benefits that exist at the effective date. Retrospective adoption is allowed. This adoption did not have any impact on the Company's financial statements.

Recently Issued Accounting Pronouncements

Qualified Affordable Housing Projects

In January 2014 the FASB issued ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects ("QAHP"), which permits companies to make an accounting policy election to account for investments in a QAHP using the proportional amortization method, if certain conditions are met. Under this method, the initial cost of the investment is amortized in proportion to the tax credits and other tax benefits received, with the net investment performance recognized in the income statement as a component of income tax expense. The new guidance also requires certain new disclosures for all QAHP investments. ASU 2014-01 is effective for annual and interim reporting periods beginning after December 15, 2014 and must be applied retrospectively to all periods presented upon adoption. The Company currently holds an investment in a QAHP that is accounted for under the equity method and does not expect the adoption to have a material impact on its financial position, results of operations or cash flows.

Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity

In April 2014, the FASB issued ASU 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity to reduce diversity in practice for reporting discontinued operations. Under the previous guidance, any component of an entity that was a reportable segment, an operating segment, a reporting unit, a subsidiary, or an asset group was eligible for discontinued operations presentation. The revised guidance only allows disposals of components of an entity that represent a strategic shift (e.g., disposal of a major geographical area, a major line of business, a major equity method investment, or other major parts of an entity) and that have a major effect on a reporting entity’s operations and financial results to be reported as discontinued operations. The revised guidance also requires expanded disclosure in the financial statements for discontinued operations as well as for disposals of significant components of an entity that do not qualify for discontinued operations presentation. The updated guidance is effective for periods beginning after December 15, 2014. The Company currently has operations that are reported as discontinued operations and does not expect the adoption of this guidance to have a material effect on its financial position, results of operations, or cash flows.