Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
White Mountains \| Restricted Share Cliff Vesting in January 2021
Target Restricted Share Activity
Issued (in shares)	13,450
White Mountains \| Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	53,755	70,620
Issued (in shares)	14,105	16,735
Vested (in shares)	(25,381)	(22,015)
Assumed forfeitures and cancellations	(969)	(5,200)
Ending of period (in shares)	41,510	60,140
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 14.3	$ 19.7
Issued	11.4	15.8
Assumed forfeitures and cancellations	(0.2)	(2.8)
Expense recognized	(3.3)	(3.6)
Unamortized grant date fair value, ending balance	$ 22.2	$ 29.1
White Mountains \| Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)	365
White Mountains \| Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)	290	16,735
Sirius Group \| Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)		2,195

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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-Publisher FASB
-Name Accounting Standards Codification
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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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