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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	Pension liability and other accumulated comprehensive items AOCI, after tax	Pension liability and other accumulated comprehensive items Total	Pension liability and other accumulated comprehensive items 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	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Common shares and paid-in surplus	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Retained earnings	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Non-controlling interest	Sale of Sirius Group Common shares and paid-in surplus	Sale of Sirius Group Retained earnings	Sale of Sirius Group AOCI, after tax	Sale of Sirius Group Total	Sale of Sirius Group Non-controlling interest	Sale of Sirius Group Total Equity	Star & Shield Insurance Exchange Common shares and paid-in surplus	Star & Shield Insurance Exchange Retained earnings	Star & Shield Insurance Exchange AOCI, after tax	Star & Shield Insurance Exchange Total	Star & Shield Insurance Exchange Non-controlling interest	Star & Shield Insurance Exchange 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	$ 581.1	0.0	604.7	0.0	604.7	(23.6)	581.1
Net change in foreign currency translation and other								$ 0.0	$ 0.0	$ 0.4	$ 0.4	$ 0.1	$ 0.5
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of Sirius Group														$ 2.9	$ 2.9	$ 2.9							$ 0.0	$ 0.0	$ 0.0
Total comprehensive income (loss)	608.1	0.0	604.7	3.3	608.0	(23.5)	584.5
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	(17.7)	(17.7)
Repurchases and retirements of common shares		(147.9)	(575.9)	0.0	(723.8)	(5.2)	(729.0)
Issuances of common shares		1.7	0.0	0.0	1.7	0.0	1.7
Dilution from equity-based units of subsidiary																	$ (4.1)	$ 0.0	$ 0.0	$ (4.1)	$ 4.7	$ 0.6
Capital contributions from BAM members, net of tax	(25.7)	0.0	0.0	0.0	0.0	18.7	18.7
Amortization of restricted share awards		10.6	0.0	0.0	10.6	0.8	11.4
Deconsolidation of non-controlling interests associated with the sale of Star & Shield																										$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (238.3)	$ (238.3)	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (4.4)	$ (4.4)
Net contributions from non-controlling interests		(1.7)	0.0	0.0	(1.7)	0.0	(1.7)
Ending Balances at Sep. 30, 2017		669.3	2,800.8	(1.3)	3,468.8	(131.6)	3,337.2
Beginning 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	(50.9)	0.0	(3.7)	0.0	(3.7)	(47.2)	(50.9)
Net change in foreign currency translation and other								$ 0.0	$ 0.0	$ (1.5)	$ (1.5)	$ 0.0	$ (1.5)
Total comprehensive income (loss)	(5.4)	0.0	(3.7)	(1.5)	(5.2)	(47.2)	(52.4)
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	(6.0)	(6.0)
Repurchases and retirements of common shares		(104.6)	(408.4)	0.0	(513.0)	0.0	(513.0)
Issuances of common shares		2.0	0.0	0.0	2.0	0.0	2.0
Recognition of equity-based units of subsidiary		4.5	0.0	0.0	4.5	2.3	6.8
Dilution from equity-based units of subsidiary		(1.1)	0.0	0.0	(1.1)	1.1	0.0
Capital contributions from BAM members, net of tax	(26.9)	0.0	0.0	0.0	0.0	22.9	22.9
Amortization of restricted share awards		9.7	0.0	0.0	9.7	0.0	9.7
Noncontrolling interest from acquisition		0.0	0.0	0.0	0.0	14.2	14.2
Acquisition from non-controlling interest		(1.6)	0.0	0.0	(1.6)	0.0	(1.6)
Ending Balances at Sep. 30, 2018	$ 2,839.6	$ 579.5	$ 2,407.3	$ (2.8)	$ 2,984.0	$ (144.4)	$ 2,839.6

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- Definition

Noncontrolling Interest, Recognition of Equity-Based Shares

+ References

No definition available.

+ Details

Name:	wtm_NoncontrollingInterestRecognitionofEquityBasedShares
Namespace Prefix:	wtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration