CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Unaudited) - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity	Foreign Currency Gain (Loss) Common shares and paid-in surplus	Foreign Currency Gain (Loss) Retained earnings	Foreign Currency Gain (Loss) AOCI, after tax	Foreign Currency Gain (Loss) Total	Foreign Currency Gain (Loss) Non-controlling interest	Foreign Currency Gain (Loss) Total Equity	OneBeacon Common shares and paid-in surplus	OneBeacon Retained earnings	OneBeacon AOCI, after tax	OneBeacon Total	OneBeacon Non-controlling interest	OneBeacon Total Equity	Sale of Tranzact Common shares and paid-in surplus	Sale of Tranzact Retained earnings	Sale of Tranzact AOCI, after tax	Sale of Tranzact Total	Sale of Tranzact Non-controlling interest	Sale of Tranzact Total Equity
Beginning Balances at Dec. 31, 2016		$ 810.7	$ 2,776.6	$ (4.6)	$ 3,582.7	$ 133.3	$ 3,716.0
Increase (Decrease) in Shareholders' Equity
Net loss (income)	$ 28.0	0.0	29.0	0.0	29.0	(1.0)	28.0
Net change in foreign currency translation and benefit plan assets and obligations								$ 0.0	$ 0.0	$ 0.2	$ 0.2	$ 0.0	$ 0.2
Total comprehensive income (loss)	29.2	0.0	29.0	0.2	29.2	(1.0)	28.2
Dividends declared on common shares		0.0	(4.6)	0.0	(4.6)	0.0	(4.6)
Dividends to non-controlling interests		0.0	0.0	0.0	0.0	(6.6)	(6.6)
Repurchases and retirements of common shares		(1.4)	(5.1)	0.0	(6.5)	(1.1)	(7.6)
Dilution from equity-based units of subsidiary														$ (4.1)	$ 0.0	$ 0.0	$ (4.1)	$ 4.1	$ 0.0
Capital contributions from BAM members, net of tax	(9.6)	0.0	0.0	0.0	0.0	7.0	7.0
Amortization of restricted share awards		2.6	0.0	0.0	2.6	0.2	2.8
Deconsolidation of non-controlling interests associated with the sale of Tranzact																				$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (4.4)	$ (4.4)
Ending Balances at Mar. 31, 2017	3,730.8	807.8	2,795.9	(4.4)	3,599.3	131.5	3,730.8
Beginning Balances at Dec. 31, 2016		810.7	2,776.6	(4.6)	3,582.7	133.3	3,716.0
Increase (Decrease) in Shareholders' Equity
Recognition of equity-based units of subsidiary						(2.3)	(6.4)
Dilution from equity-based units of subsidiary						0.9	0.0
Ending Balances at Dec. 31, 2017	3,360.8	670.6	2,823.2	(1.3)	3,492.5	(131.7)	3,360.8
Increase (Decrease) in Shareholders' Equity
Net loss (income)	(66.6)	0.0	(48.0)	0.0	(48.0)	(18.6)	(66.6)
Net change in foreign currency translation and benefit plan assets and obligations								$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Total comprehensive income (loss)	(48.0)	0.0	(48.0)	0.0	(48.0)	(18.6)	(66.6)
Dividends declared on common shares		0.0	(3.8)	0.0	(3.8)	0.0	(3.8)
Dividends to non-controlling interests		0.0	0.0	0.0	0.0	(0.3)	(0.3)
Repurchases and retirements of common shares		(1.9)	(6.4)	0.0	(8.3)	0.0	(8.3)
Recognition of equity-based units of subsidiary		(4.1)	0.0	0.0	(4.1)
Dilution from equity-based units of subsidiary		(0.9)	0.0	0.0	(0.9)
Capital contributions from BAM members, net of tax	(4.9)	0.0	0.0	0.0	0.0	4.2	4.2
Amortization of restricted share awards		3.4	0.0	0.0	3.4	0.0	3.4
Ending Balances at Mar. 31, 2018	$ 3,295.8	$ 675.3	$ 2,765.0	$ (1.3)	$ 3,439.0	$ (143.2)	$ 3,295.8