Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 7.3
White Mountains \| W T M Incentive Plan \| Restricted Share Cliff Vesting in January 2019 [Member]
Target Restricted Share Activity
Issued (in shares)	21,215
White Mountains \| W T M Incentive Plan \| Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)		22,890
White Mountains \| W T M Incentive Plan \| Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	70,675	83,314
Issued (in shares)	21,215	22,890
Vested (in shares)	(24,620)	(35,079)
Assumed forfeitures and cancellations	(800)	0
Ending of period (in shares)	66,470	71,125
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 15.7	$ 14.3
Issued	16.3	15.2
Assumed forfeitures and cancellations	0.2	0.0
Expense recognized	(3.0)	(3.0)
Unamortized grant date fair value, ending balance	$ 29.2	$ 26.5
Sirius Group \| W T M Incentive Plan \| Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)	2,512	3,572

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Unrecognized cost of unvested share-based compensation awards.

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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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