Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted Share Cliff Vesting in January 2017 [Member]
Target Performance Share activity
Issued (in shares)			23,440
W T M Incentive and Phantom Plan [Member] \| Restricted Share Cliff Vesting in January 2019 [Member]
Target Performance Share activity
Issued (in shares)	24,615
W T M Incentive and Phantom Plan [Member] \| Restricted Share Cliff Vesting in January 2018 [Member]
Target Performance Share activity
Issued (in shares)	750	23,640
White Mountains \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	70,675	83,314	94,130
Issued (in shares)	25,365	23,640	23,440
Vested (in shares)	(24,620)	(36,279)	(33,205)
Assumed forfeitures and cancellations (in shares)	(800)	0	(1,051)
End of period (3)	70,620	70,675	83,314
Unamortized grant date fair value
Beginning balance	$ 15.7	$ 14.3	$ 17.0
Issued	20.2	15.7	13.1
Assumed forfeitures and cancellations	(0.3)	0.0	(0.5)
Expense recognized	(15.9)	(14.3)	(15.3)
Ending balance	$ 19.7	$ 15.7	$ 14.3
White Mountains \| W T M Incentive Plan \| Performance Shares
Target Performance Share activity
Beginning of period	93,654	111,257	108,605
Assumed forfeitures and cancellations (in shares)	378	(3,839)	(3,208)
End of period (3)	80,353	93,654	111,257
Unamortized grant date fair value
Assumed forfeitures and cancellations	$ 0.5	$ (0.3)	$ (0.1)
Expense recognized	(25.2)	$ (44.4)	$ (14.1)
Ending balance	$ 23.1
Sirius Group \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	9,205	12,477
End of period (3)	5,235	9,205	12,477
Sirius Group \| W T M Incentive Plan \| Performance Shares
Target Performance Share activity
Beginning of period	10,826	12,607
End of period (3)	7,315	10,826	12,607
Range of Performance Cycle from 2011 to 2013 [Member] \| Performance Shares
Target Performance Share activity
Vested (in shares)			(3,570)
Range of Performance Cycle from 2011 to 2013 [Member] \| W T M Incentive and Phantom Plan [Member] \| Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage			131.50%
Range of Performance Cycle from 2011 to 2013 [Member] \| Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Percentage of payout for minimum growth target percentage			37.10%
Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive Plan \| Performance Shares
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Number of share instruments newly issued under a share-based compensation plan.

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The total reduction to unamortized compensation cost for performance shares forfeited during the period.

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The total fair value (on the date of issue) for restricted share awards issued during the period.

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- Definition

Represents the lpercentage of target achieved, high end of range.

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Represents the lpercentage of target achieved, low end of range.

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