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Financial Statement Revisions - Statements of Changes in Equity (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Shareholders' Equity
Beginning Balances	$ 3,360.8
Net loss (income)	(66.6)	$ 28.0
Net (loss) income attributable to White Mountains’s common shareholders	(48.0)	29.0
Ending Balances	3,295.8	3,730.8
As previously reported
Increase (Decrease) in Shareholders' Equity
Net loss (income)		32.9
Net (loss) income attributable to White Mountains’s common shareholders		34.2
Ending Balances		3,757.5
Adjustments
Increase (Decrease) in Shareholders' Equity
Net loss (income)		(4.9)
Net (loss) income attributable to White Mountains’s common shareholders		(5.2)	$ (5.2)
Ending Balances		(26.7)
Common shares and paid-in surplus
Increase (Decrease) in Shareholders' Equity
Beginning Balances	670.6	810.7
Repurchases and retirements of common shares	(1.9)	(1.4)
Net loss (income)	0.0	0.0
Dividends declared on common shares	0.0	0.0
Ending Balances	675.3	807.8	810.7
Common shares and paid-in surplus \| As previously reported
Increase (Decrease) in Shareholders' Equity
Ending Balances		807.1
Common shares and paid-in surplus \| Adjustments
Increase (Decrease) in Shareholders' Equity
Ending Balances		0.7
Retained earnings
Increase (Decrease) in Shareholders' Equity
Beginning Balances	2,823.2	2,776.6
Repurchases and retirements of common shares	(6.4)	(5.1)
Net loss (income)	(48.0)	29.0
Dividends declared on common shares	(3.8)	(4.6)
Ending Balances	2,765.0	2,795.9	2,776.6
Retained earnings \| As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances		2,797.2
Repurchases and retirements of common shares		(5.1)
Net loss (income)		34.2
Dividends declared on common shares		(4.6)
Ending Balances		2,821.7	2,797.2
Retained earnings \| Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances		(20.6)
Repurchases and retirements of common shares		0.0
Dividends declared on common shares		0.0
Ending Balances		(25.8)	(20.6)
AOCI, after tax
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(1.3)	(4.6)
Repurchases and retirements of common shares	0.0	0.0
Net loss (income)	0.0	0.0
Dividends declared on common shares	0.0	0.0
Ending Balances	(1.3)	(4.4)	(4.6)
AOCI, after tax \| As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances		(4.6)
Ending Balances		(3.7)	(4.6)
AOCI, after tax \| Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances		0.0
Ending Balances		(0.7)	0.0
Total
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,492.5	3,582.7
Repurchases and retirements of common shares	(8.3)	(6.5)
Net loss (income)	(48.0)	29.0
Dividends declared on common shares	(3.8)	(4.6)
Ending Balances	3,439.0	3,599.3	3,582.7
Total \| As previously reported
Increase (Decrease) in Shareholders' Equity
Ending Balances		3,625.1
Total \| Adjustments
Increase (Decrease) in Shareholders' Equity
Ending Balances		(25.8)
Non-controlling interest
Increase (Decrease) in Shareholders' Equity
Beginning Balances	(131.7)	133.3
Repurchases and retirements of common shares	0.0	(1.1)
Net loss (income)	(18.6)	(1.0)
Dividends declared on common shares	0.0	0.0
Other changes in non-controlling interests		(0.8)
Ending Balances	(143.2)	131.5	133.3
Non-controlling interest \| As previously reported
Increase (Decrease) in Shareholders' Equity
Beginning Balances		133.9
Net loss (income)		(1.3)
Other changes in non-controlling interests		(0.2)
Ending Balances		132.4	133.9
Non-controlling interest \| Adjustments
Increase (Decrease) in Shareholders' Equity
Beginning Balances		(0.6)
Net loss (income)		0.3
Other changes in non-controlling interests		(0.6)
Ending Balances		(0.9)	(0.6)
Total Equity
Increase (Decrease) in Shareholders' Equity
Beginning Balances	3,360.8	3,716.0
Repurchases and retirements of common shares	(8.3)	(7.6)
Net loss (income)	(66.6)	28.0
Dividends declared on common shares	(3.8)	(4.6)
Ending Balances	3,295.8	3,730.8	$ 3,716.0
Total Equity \| As previously reported
Increase (Decrease) in Shareholders' Equity
Ending Balances		3,757.5
Total Equity \| Adjustments
Increase (Decrease) in Shareholders' Equity
Ending Balances		(26.7)
Foreign Currency Gain (Loss) \| Common shares and paid-in surplus
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.0
Foreign Currency Gain (Loss) \| Retained earnings
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.0
Foreign Currency Gain (Loss) \| AOCI, after tax
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.2
Foreign Currency Gain (Loss) \| AOCI, after tax \| As previously reported
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax		0.9
Foreign Currency Gain (Loss) \| AOCI, after tax \| Adjustments
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax		(0.7)
Foreign Currency Gain (Loss) \| Total
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.2
Foreign Currency Gain (Loss) \| Non-controlling interest
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	0.0	0.0
Foreign Currency Gain (Loss) \| Total Equity
Increase (Decrease) in Shareholders' Equity
Change in foreign currency translation, net of tax	$ 0.0	$ 0.2