CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Cash Reclassified from Assets Held-for-sale	$ 0.6	$ 2.7	$ 0.6	$ 2.7	$ 1.2	$ 2.1
Cash flows from operations:
Net income	362.5	3.4	381.8	80.4
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment gains	(28.1)	15.0	(57.6)	(10.0)
Deferred income tax expense (benefit)			12.8	(6.3)
Net loss (income) from discontinued operations			6.9	(62.9)
Net gain on sale of discontinued operations	(366.6)	0.1	(366.6)	(7.9)
Net gain on sale of unconsolidated subsidiaries			0.0	(16.1)
Amortization and depreciation			22.5	20.9
Undistributed equity in earnings from unconsolidated affiliates, net of tax	0.0	(6.8)	0.0	(14.1)
Other operating items:
Net change in loss and loss adjustment expense reserves			(13.5)	7.0
Net change in reinsurance recoverable on paid and unpaid losses			10.0	5.8
Net change in unearned insurance premiums			8.9	39.0
Net change in deferred acquisition costs			(6.3)	(10.0)
Net change in funds held by ceding entities			0.0	(12.2)
Net change in ceded unearned premiums			(5.0)	(25.4)
Net change in funds held under insurance treaties			3.2	21.6
Net change in insurance premiums receivable			(19.5)	(52.5)
Net change in ceded reinsurance payable			(6.8)	34.6
Net change in restricted cash			5.8	2.8
Net change in other assets and liabilities, net			(120.7)	(17.5)
Net cash used for operations - continuing operations			(144.1)	(22.8)
Net cash used for operations			(187.2)	(35.7)
Cash flows from investing activities:
Net change in short-term investments			(207.7)	(49.7)
Sales of fixed maturity and convertible investments			1,453.5	680.8
Maturities, calls and paydowns of fixed maturity and convertible investments			316.5	136.7
Sales of common equity securities			851.3	306.8
Distributions and redemptions of other long-term investments			10.1	49.1
Sales of consolidated and unconsolidated subsidiaries, net of cash sold			2,248.5	24.0
Purchases of other long-term investments			(13.3)	(28.6)
Purchases of common equity securities			(141.4)	(275.1)
Purchases of fixed maturity and convertible investments			(3,467.0)	(953.6)
Purchases of consolidated and unconsolidated subsidiaries, net of cash acquired			(8.1)	(2.4)
Net change in unsettled investment purchases and sales			90.3	56.2
Net acquisitions of property and equipment			(3.7)	54.9
Net cash provided from investing activities - continuing operations			719.3	6.4
Net cash provided from investing activities			948.9	65.7
Cash flows from financing activities:
Draw down of debt and revolving line of credit			102.5	15.0
Repayment of debt and revolving line of credit			(150.5)	(15.0)
Proceeds from Issuance of Common Stock			3.7	0.0
Payments on capital lease obligation			(1.0)	(2.8)
Cash dividends paid to the Company’s common shareholders			(5.4)	(6.0)
Cash dividends paid to OneBeacon Ltd.’s non-controlling common shareholders			(9.5)	(9.9)
Common shares repurchased			(541.5)	(17.1)
OneBeacon Ltd. common shares repurchased and retired			(11.5)	(1.6)
Distribution to non-controlling interest shareholders			(0.7)	(1.0)
Contributions to discontinued operations			(3.0)	(6.1)
Acquisition of additional shares from non-controlling interest			0.0	(9.1)
Payments of contingent consideration related to purchases of consolidated subsidiaries			(7.8)	0.0
Capital contributions from BAM members			16.7	11.5
Net cash used for financing activities - continuing operations			(608.0)	(42.1)
Net cash used for financing activities			(599.0)	(44.1)
Effect of Exchange Rate on Cash and Cash Equivalents			0.0	0.0
Net change in cash during the period - continuing operations			(32.8)	(58.5)
Cash balances at beginning of period			167.2	232.9	232.9
Cash balances at end of period	135.0	173.8	135.0	173.8	167.2
Supplemental cash flows information:
Interest paid			(0.5)	(73.3)
Net income tax refund from national governments			13.5	7.5
Discontinued Operations [Member]
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment gains	(7.3)	46.1	1.5	(32.3)
Net loss (income) from discontinued operations	9.1	0.9	7.9	(82.8)
Net gain on sale of discontinued operations	8.0	0.4	6.9	(62.9)
Other operating items:
Net cash used for operations - discontinued operations			(43.1)	(12.9)
Cash flows from investing activities:
Sales of common equity securities			(409.7)	7.3
Net change in cash during the period			195.5	42.3
Net cash provided from investing activities - discontinued operations			229.6	59.3
Cash flows from financing activities:
Net cash provided from (used for) financing activities - discontinued operations			9.0	(2.0)
Cash balances at beginning of period			148.9	114.5	114.5
Cash balances at end of period	$ 1.3	$ 156.8	1.3	156.8	$ 148.9
Supplemental cash flows information:
Net income tax refund from national governments			$ 31.8	$ 24.6