Consolidating Financial Information (Consolidating Statement of Cash Flows) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	$ 47.4	$ 68.8
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	0	0
Net loss from discontinued operations	0.5	(0.5)
Net realized and change in unrealized investment gains	(18.9)	(28.4)
Net realized gain on sale of business	0	(23.0)
Deferred income tax expense	10.9	22.4
Dividends received from subsidiaries	0	0
Other operating items:
Net change in loss and LAE reserves	15.5	10.3
Net change in unearned premiums	37.3	(22.8)
Net change in premiums receivable	(21.0)	(1.8)
Net change in reinsurance recoverable on paid and unpaid losses	6.8	2.4
Net change in other assets and liabilities	(59.5)	(11.0)
Net cash provided from operations—continuing operations	19.0	16.4
Net cash used for operations—discontinued operations	(14.6)	(40.7)
Net cash provided from (used for) operations	4.4	(24.3)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	4.3	14.8
Maturities of fixed maturity investments	115.7	103.7
Sales of fixed maturity investments	324.6	632.7
Sales of common equity securities	24.3	89.7
Sales of convertible fixed maturity investments	6.8	29.6
Distributions and redemptions of other investments	1.6	18.6
Purchases of fixed maturity investments	(466.0)	(674.2)
Purchases of common equity securities	(26.0)	(102.3)
Purchases of convertible fixed maturity investments	(3.8)	(5.0)
Contributions for other investments	(2.3)	(16.0)
Proceeds from sale of business	0	31.3
Net change in unsettled investment purchases and sales	10.6	(18.3)
Net acquisitions of property and equipment	(0.6)	(3.0)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.		0
Net cash provided from investing activities—continuing operations	(10.8)	101.6
Net cash provided from investing activities—discontinued operations	0	0
Net cash provided from investing activities	(10.8)	101.6
Cash flows from financing activities:
Cash dividends paid to common shareholders	(20.0)	(20.0)
Cash dividends paid to parent	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	(1.8)
Proceeds on sale of OB Holdings Lux	(1.8)	0
Net cash used for financing activities—continuing operations	(23.1)	(21.8)
Net cash used for financing activities—discontinued operations	0	0
Net cash used for financing activities	(23.1)	(21.8)
Net increase in cash during period	(29.5)	55.5
Cash balance at beginning of period	168.1	43.9
Cash balance at end of period	138.6	99.4
Proceeds from Contributed Capital		0
Repayments of Long-term Capital Lease Obligations	(1.3)	(1.8)
The Company (guarantor)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	47.0	68.4
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	(48.6)	(69.3)
Net loss from discontinued operations	0	0
Net realized and change in unrealized investment gains	0	0
Net realized gain on sale of business		0
Deferred income tax expense	0	0
Dividends received from subsidiaries	10.0	22.0
Other operating items:
Net change in loss and LAE reserves	0	0
Net change in unearned premiums	0	0
Net change in premiums receivable	0	0
Net change in reinsurance recoverable on paid and unpaid losses	0	0
Net change in other assets and liabilities	2.4	1.1
Net cash provided from operations—continuing operations	10.8	22.2
Net cash used for operations—discontinued operations	0	0
Net cash provided from (used for) operations	10.8	22.2
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	11.0	(2.2)
Maturities of fixed maturity investments	0	0
Sales of fixed maturity investments	0	0
Sales of common equity securities	0	0
Sales of convertible fixed maturity investments	0	0
Distributions and redemptions of other investments	0	0
Purchases of fixed maturity investments	0	0
Purchases of common equity securities	0	0
Purchases of convertible fixed maturity investments	0	0
Contributions for other investments	0	0
Proceeds from sale of business		0
Net change in unsettled investment purchases and sales	0	0
Net acquisitions of property and equipment	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.		0
Net cash provided from investing activities—continuing operations	11.0	(2.2)
Net cash provided from investing activities—discontinued operations	0	0
Net cash provided from investing activities	11.0	(2.2)
Cash flows from financing activities:
Cash dividends paid to common shareholders	(20.0)	(20.0)
Cash dividends paid to parent	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	(1.8)
Net cash used for financing activities—continuing operations	(21.8)	(20.0)
Net cash used for financing activities—discontinued operations	0	0
Net cash used for financing activities	(21.8)	(20.0)
Net increase in cash during period	0	0
Cash balance at beginning of period	0	0
Cash balance at end of period	0	0
Proceeds from Contributed Capital		0
Repayments of Long-term Capital Lease Obligations	0	0
Non-guarantor subsidiaries
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	51.3	70.9
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	0	0
Net loss from discontinued operations	0.5	(0.5)
Net realized and change in unrealized investment gains	(19.4)	(28.7)
Net realized gain on sale of business		(23.0)
Deferred income tax expense	11.1	22.4
Dividends received from subsidiaries	0	0
Other operating items:
Net change in loss and LAE reserves	15.5	10.3
Net change in unearned premiums	37.3	(22.8)
Net change in premiums receivable	(21.0)	(1.8)
Net change in reinsurance recoverable on paid and unpaid losses	6.8	2.4
Net change in other assets and liabilities	(63.7)	(17.1)
Net cash provided from operations—continuing operations	18.4	12.1
Net cash used for operations—discontinued operations	(14.6)	(40.7)
Net cash provided from (used for) operations	3.8	(28.6)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(3.3)	(17.8)
Maturities of fixed maturity investments	115.7	103.1
Sales of fixed maturity investments	324.5	567.0
Sales of common equity securities	24.3	89.7
Sales of convertible fixed maturity investments	9.8	29.6
Distributions and redemptions of other investments	1.6	18.6
Purchases of fixed maturity investments	(466.0)	(576.1)
Purchases of common equity securities	(26.0)	(102.3)
Purchases of convertible fixed maturity investments	(3.8)	(5.0)
Contributions for other investments	(2.3)	(16.0)
Proceeds from sale of business		31.3
Net change in unsettled investment purchases and sales	10.6	(18.3)
Net acquisitions of property and equipment	(0.6)	(3.0)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.		30.0
Net cash provided from investing activities—continuing operations	(15.5)	130.8
Net cash provided from investing activities—discontinued operations	0	0
Net cash provided from investing activities	(15.5)	130.8
Cash flows from financing activities:
Cash dividends paid to common shareholders	0	0
Cash dividends paid to parent	(15.0)	(45.0)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0
Net cash used for financing activities—continuing operations	(16.3)	(46.8)
Net cash used for financing activities—discontinued operations	0	0
Net cash used for financing activities	(16.3)	(46.8)
Net increase in cash during period	(28.0)	55.4
Cash balance at beginning of period	166.6	43.9
Cash balance at end of period	138.6	99.3
Proceeds from Contributed Capital		0
Repayments of Long-term Capital Lease Obligations	(1.3)	(1.8)
OBH (issuer)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	39.1	57.5
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	(41.1)	(58.5)
Net loss from discontinued operations	0	0
Net realized and change in unrealized investment gains	0	(0.1)
Net realized gain on sale of business		0
Deferred income tax expense	0	0.2
Dividends received from subsidiaries	5.0	23.0
Other operating items:
Net change in loss and LAE reserves	0	0
Net change in unearned premiums	0	0
Net change in premiums receivable	0	0
Net change in reinsurance recoverable on paid and unpaid losses	0	0
Net change in other assets and liabilities	1.8	5.0
Net cash provided from operations—continuing operations	4.8	27.1
Net cash used for operations—discontinued operations	0	0
Net cash provided from (used for) operations	4.8	27.1
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(3.4)	34.8
Maturities of fixed maturity investments	0	0.6
Sales of fixed maturity investments	0.1	101.4
Sales of common equity securities	0	0
Sales of convertible fixed maturity investments	(3.0)	0
Distributions and redemptions of other investments	0	0
Purchases of fixed maturity investments	0	(133.8)
Purchases of common equity securities	0	0
Purchases of convertible fixed maturity investments	0	0
Contributions for other investments	0	0
Proceeds from sale of business		0
Net change in unsettled investment purchases and sales	0	0
Net acquisitions of property and equipment	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.		0
Net cash provided from investing activities—continuing operations	(6.3)	3.0
Net cash provided from investing activities—discontinued operations	0	0
Net cash provided from investing activities	(6.3)	3.0
Cash flows from financing activities:
Cash dividends paid to common shareholders	0	0
Cash dividends paid to parent	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0
Net cash used for financing activities—continuing operations	0	(30.0)
Net cash used for financing activities—discontinued operations	0	0
Net cash used for financing activities	0	(30.0)
Net increase in cash during period	(1.5)	0.1
Cash balance at beginning of period	1.5	0
Cash balance at end of period	0	0.1
Proceeds from Contributed Capital		(30.0)
Repayments of Long-term Capital Lease Obligations	0	0
Consolidating adjustments
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	(90.0)	(128.0)
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	89.7	127.8
Net loss from discontinued operations	0	0
Net realized and change in unrealized investment gains	0.5	0.4
Net realized gain on sale of business		0
Deferred income tax expense	(0.2)	(0.2)
Dividends received from subsidiaries	(15.0)	(45.0)
Other operating items:
Net change in loss and LAE reserves	0	0
Net change in unearned premiums	0	0
Net change in premiums receivable	0	0
Net change in reinsurance recoverable on paid and unpaid losses	0	0
Net change in other assets and liabilities	0	0
Net cash provided from operations—continuing operations	(15.0)	(45.0)
Net cash used for operations—discontinued operations	0	0
Net cash provided from (used for) operations	(15.0)	(45.0)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	0	0
Maturities of fixed maturity investments	0	0
Sales of fixed maturity investments	0	(35.7)
Sales of common equity securities	0	0
Sales of convertible fixed maturity investments	0	0
Distributions and redemptions of other investments	0	0
Purchases of fixed maturity investments	0	35.7
Purchases of common equity securities	0	0
Purchases of convertible fixed maturity investments	0	0
Contributions for other investments	0	0
Proceeds from sale of business		0
Net change in unsettled investment purchases and sales	0	0
Net acquisitions of property and equipment	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.		(30.0)
Net cash provided from investing activities—continuing operations	0	(30.0)
Net cash provided from investing activities—discontinued operations	0	0
Net cash provided from investing activities	0	(30.0)
Cash flows from financing activities:
Cash dividends paid to common shareholders	0	0
Cash dividends paid to parent	15.0	45.0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0
Net cash used for financing activities—continuing operations	15.0	75.0
Net cash used for financing activities—discontinued operations	0	0
Net cash used for financing activities	15.0	75.0
Net increase in cash during period	0	0
Cash balance at beginning of period	0	0
Cash balance at end of period	0	0
Proceeds from Contributed Capital		30.0
Repayments of Long-term Capital Lease Obligations	$ 0	$ 0