CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCL, after-tax	Parent	Non-controlling interests	Total equity	Sale of Sirius Group	Sale of Sirius Group Non-controlling interests	Sale of Sirius Group Total equity	Tranzact Non-controlling interests	Tranzact Total equity	OneBeacon Non-controlling interests	OneBeacon Total equity	Star & Shield LLC Non-controlling interests	Star & Shield LLC Total equity	BAM Non-controlling interests	BAM Total equity	WOBI Common shares and paid-in surplus	WOBI Parent	WOBI Total equity
Beginning balance at Dec. 31, 2015		$ 978.2	$ 3,075.0	$ (149.9)	$ 3,903.3	$ 454.3	$ 4,357.6
Increase (Decrease) in Shareholders' Equity
Net (loss) income	$ 409.0		401.8		401.8	7.2	409.0
Net change in foreign currency translation and interest rate swap				31.4	31.4	0.3	31.7
Other comprehensive (loss) income, net of tax	(0.7)			113.9	113.9		113.9
Net change in pension liability and other accumulated comprehensive items								$ 113.3
Comprehensive income (loss)	547.4		401.8	145.3	547.1	7.5	554.6
Dividends declared on common shares			(5.4)		(5.4)		(5.4)
Dividends to non-controlling interests						(22.7)	(22.7)
Issuances of common shares		9.1			9.1		9.1
Repurchases and retirements of common shares	(887.2)	(192.4)	(694.8)		(887.2)		(887.2)
Capital contributions from non-controlling interests						27.3	27.3
Deconsolidation of non-controlling interests									$ (250.0)	$ (250.0)	$ (78.4)	$ (78.4)
Acquisition of noncontrolling interest		(2.7)			(2.7)	(8.8)	(11.5)
Acquisition of subsidiary						3.3	3.3
Amortization of restricted share and option awards		18.5			18.5	0.8	19.3
Ending 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 (loss) income	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
Other comprehensive (loss) income, net of tax	0.3			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)
Issuance of shares of non-controlling interests		(4.1)			(4.1)	5.2	1.1
Capital contributions from non-controlling interests		(4.6)			(4.6)	3.0	(1.6)										$ 27.2	$ 27.2
Deconsolidation of non-controlling interests													$ (238.3)	$ (238.3)	$ (4.4)	$ (4.4)
Amortization of restricted share and option awards		14.8			14.8	0.8	15.6
Ending balance at Dec. 31, 2017	3,492.5	670.6	2,823.2	(1.3)	3,492.5	(131.7)	3,360.8
Increase (Decrease) in Shareholders' Equity
Net (loss) income	(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)
Other comprehensive (loss) income, net of tax	(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	(519.4)	(105.8)	(413.3)		(519.1)		(519.1)
Capital contributions from non-controlling interests																	$ 45.0	$ 45.0
Noncontrolling Interest, Recognition of Equity-Based Shares		7.4	0.0	0.0	7.4	4.3	11.7
Noncontrolling Interest, Dilution from Equity-based Shares		(1.5)	0.0	0.0	(1.5)	1.5	0.0
Noncontrolling Interest, Decrease from Purchase of Interests																			$ (1.7)	$ (1.7)	$ (1.7)
Acquisition of subsidiary						14.5	14.5
Amortization of restricted share and option awards		13.0			13.0		13.0
Ending balance at Dec. 31, 2018	$ 2,843.1	$ 584.0	$ 2,264.9	$ (5.8)	$ 2,843.1	$ (124.9)	$ 2,718.2