Consolidating Financial Information (Consolidating Statement of Operations and Comprehensive (Loss)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Revenues
Earned premiums	$ 276.5	$ 286.5
Net investment income (expense)	10.0	9.4
Net realized and change in unrealized investment gains	18.9	28.4
Net other revenues	1.0	24.0
Total revenues	306.4	348.3
Expenses
Loss and loss adjustment expenses	149.4	148.9
Policy acquisition expenses	46.7	54.8
Other underwriting expenses	49.4	49.3
General and administrative expenses	3.3	4.0
Interest expense	3.2	3.2
Total expenses	252.0	260.2
Pre-tax (loss) income from continuing operations	54.4	88.1
Income tax expense	(6.5)	(19.8)
Net income from continuing operations	47.9	68.3
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	(0.5)	0.5
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	47.4	68.8
Equity in earnings of subsidiaries, net of tax	0	0
Net income, including noncontrolling interests	47.4	68.8
Less: Net income attributable to noncontrolling interests	(0.4)	(0.4)
Net income attributable to OneBeacon's common shareholders	47.0	68.4
Net change in benefit plan assets and obligations, net of tax	0	0.1
Comprehensive income attributable to OneBeacon's common shareholders	47.0	68.5
The Company (guarantor)
Revenues
Earned premiums	0	0
Net investment income (expense)	0	0
Net realized and change in unrealized investment gains	0	0
Net other revenues	0	0
Total revenues	0	0
Expenses
Loss and loss adjustment expenses	0	0
Policy acquisition expenses	0	0
Other underwriting expenses	0	0
General and administrative expenses	1.6	1.0
Interest expense	0	0
Total expenses	1.6	1.0
Pre-tax (loss) income from continuing operations	(1.6)	(1.0)
Income tax expense	0	0.1
Net income from continuing operations	(1.6)	(0.9)
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	0	0
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	(1.6)	(0.9)
Equity in earnings of subsidiaries, net of tax	48.6	69.3
Net income, including noncontrolling interests	47.0	68.4
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to OneBeacon's common shareholders	47.0	68.4
Net change in benefit plan assets and obligations, net of tax	0	0.1
Comprehensive income attributable to OneBeacon's common shareholders	47.0	68.5
Non-guarantor subsidiaries
Revenues
Earned premiums	276.5	286.5
Net investment income (expense)	10.3	10.3
Net realized and change in unrealized investment gains	19.4	28.7
Net other revenues	1.0	24.0
Total revenues	307.2	349.5
Expenses
Loss and loss adjustment expenses	149.4	148.9
Policy acquisition expenses	46.7	54.8
Other underwriting expenses	49.4	49.3
General and administrative expenses	1.6	3.2
Interest expense	0	0
Total expenses	247.1	256.2
Pre-tax (loss) income from continuing operations	60.1	93.3
Income tax expense	(8.3)	(22.9)
Net income from continuing operations	51.8	70.4
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	(0.5)	0.5
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	51.3	70.9
Equity in earnings of subsidiaries, net of tax	0	0
Net income, including noncontrolling interests	51.3	70.9
Less: Net income attributable to noncontrolling interests	(0.4)	(0.4)
Net income attributable to OneBeacon's common shareholders	50.9	70.5
Net change in benefit plan assets and obligations, net of tax	0	0
Comprehensive income attributable to OneBeacon's common shareholders	50.9	70.5
OBH (issuer)
Revenues
Earned premiums	0	0
Net investment income (expense)	0	(0.6)
Net realized and change in unrealized investment gains	0	0.1
Net other revenues	0	0
Total revenues	0	(0.5)
Expenses
Loss and loss adjustment expenses	0	0
Policy acquisition expenses	0	0
Other underwriting expenses	0	0
General and administrative expenses	0.1	(0.2)
Interest expense	3.5	3.5
Total expenses	3.6	3.3
Pre-tax (loss) income from continuing operations	(3.6)	(3.8)
Income tax expense	1.6	2.8
Net income from continuing operations	(2.0)	(1.0)
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	0	0
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	(2.0)	(1.0)
Equity in earnings of subsidiaries, net of tax	41.1	58.5
Net income, including noncontrolling interests	39.1	57.5
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to OneBeacon's common shareholders	39.1	57.5
Net change in benefit plan assets and obligations, net of tax	0	0.1
Comprehensive income attributable to OneBeacon's common shareholders	39.1	57.6
Consolidating adjustments
Revenues
Earned premiums	0	0
Net investment income (expense)	(0.3)	(0.3)
Net realized and change in unrealized investment gains	(0.5)	(0.4)
Net other revenues	0	0
Total revenues	(0.8)	(0.7)
Expenses
Loss and loss adjustment expenses	0	0
Policy acquisition expenses	0	0
Other underwriting expenses	0	0
General and administrative expenses	0	0
Interest expense	(0.3)	(0.3)
Total expenses	(0.3)	(0.3)
Pre-tax (loss) income from continuing operations	(0.5)	(0.4)
Income tax expense	0.2	0.2
Net income from continuing operations	(0.3)	(0.2)
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	0	0
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	(0.3)	(0.2)
Equity in earnings of subsidiaries, net of tax	(89.7)	(127.8)
Net income, including noncontrolling interests	(90.0)	(128.0)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to OneBeacon's common shareholders	(90.0)	(128.0)
Net change in benefit plan assets and obligations, net of tax	0	(0.1)
Comprehensive income attributable to OneBeacon's common shareholders	$ (90.0)	$ (128.1)