CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Common Shares and Paid-in Surplus	Retained Earnings	AOCI, after tax	Parent	Non-controlling Interests	Total Equity	One Beacon Insurance Company [Member] Non-controlling Interests	One Beacon Insurance Company [Member] Total Equity	Star & Shield LLC [Member] Non-controlling Interests	Star & Shield LLC [Member] Total Equity	BAM Non-controlling Interests	BAM Total Equity	Foreign Currency Gain (Loss) AOCI, after tax	Foreign Currency Gain (Loss) Parent	Foreign Currency Gain (Loss) Non-controlling Interests	Foreign Currency Gain (Loss) Total Equity
Beginning balance 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 income (loss)	$ 593.1		627.2		627.2	(34.1)	593.1
Net change in foreign currency translation and interest rate swap														$ 0.4	$ 0.4	$ 0.1	$ 0.5
Net change in pension liability and other accumulated comprehensive items														$ 2.9	$ 2.9		$ 2.9
Comprehensive income (loss)	630.7		627.2	3.3	630.5	(34.0)	596.5
Dividends declared on common shares			(4.6)		(4.6)		(4.6)
Dividends to non-controlling interests						(19.3)	(19.3)
Issuances of common shares		1.7			1.7		1.7
Repurchases and retirements of common shares	(723.9)	(147.9)	(576.0)		(723.9)	(5.2)	(729.1)
Capital contributions from non-controlling interests		(4.6)			(4.6)	3.0	(1.6)					$ 27.2	$ 27.2
Issuance of shares of non-controlling interests		(4.1)			(4.1)	5.2	1.1
Amortization of restricted share and option awards		14.8			14.8	0.8	15.6
Deconsolidation of non-controlling interests		0.0						$ (238.3)	$ (238.3)	$ (4.4)	$ (4.4)
Ending balance at Dec. 31, 2017		670.6	2,823.2	(1.3)	3,492.5	(131.7)	3,360.8
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(191.4)		(141.2)		(141.2)	(50.2)	(191.4)
Net change in foreign currency translation and interest rate swap				(4.5)	(4.5)	(0.3)	(4.8)
Comprehensive income (loss)	(146.0)		(141.2)	(4.5)	(145.7)	(50.5)	(196.2)
Dividends declared on common shares			(3.8)		(3.8)		(3.8)
Dividends to non-controlling interests						(8.0)	(8.0)
Issuances of common shares		2.0			2.0		2.0
Repurchases and retirements of common shares	(519.1)	(105.8)	(413.3)		(519.1)	0.0	(519.1)
Capital contributions from non-controlling interests												45.0	45.0
Amortization of restricted share and option awards		13.0			13.0	0.0	13.0
Acquisition of non-controlling interests		7.4			7.4	4.3	11.7
Dilution from equity units of subsidiary		(1.5)			(1.5)	1.5	0.0
Acquisition of non-controlling interests						14.5	14.5
Acquisition of subsidiary		(1.7)			(1.7)		(1.7)
Ending balance at Dec. 31, 2018	2,843.1	584.0	2,264.9	(5.8)	2,843.1	(124.9)	2,718.2
Increase (Decrease) in Shareholders' Equity
Net income (loss)	376.6		414.5		414.5	(37.9)	376.6
Net change in foreign currency translation and interest rate swap				(1.4)	(1.4)	0.0	(1.4)
Comprehensive income (loss)	413.1		414.5	(1.4)	413.1	(37.9)	375.2
Dividends declared on common shares			(3.2)		(3.2)		(3.2)
Dividends to non-controlling interests						(2.0)	(2.0)
Issuances of common shares		2.2			2.2		2.2
Repurchases and retirements of common shares	(4.9)	(1.1)	(3.8)		(4.9)		(4.9)
Capital contributions from non-controlling interests		(0.9)			(0.9)	2.4	1.5					$ 57.6	$ 57.6
Amortization of restricted share and option awards		10.6			10.6		10.6
Acquisition of non-controlling interests		1.5			1.5	0.0	1.5
Deconsolidation of non-controlling interests						(13.8)	(13.8)
Acquisition of non-controlling interests		0.0			0.0	1.8	1.8
Ending balance at Dec. 31, 2019	$ 3,261.5	$ 596.3	$ 2,672.4	$ (7.2)	$ 3,261.5	$ (116.8)	$ 3,144.7