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Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 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)	41,510	53,815	53,755	70,620
Issued (in shares)	0	0	14,105	17,785
Vested (in shares)	0	(260)	(25,381)	(28,846)
Assumed forfeitures and cancellations	0	0	(969)	(6,004)
Ending of period (in shares)	41,510	53,555	41,510	53,555
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 19.0	$ 23.5	$ 14.3	$ 19.7
Issued	0.0	0.0	11.4	16.7
Assumed forfeitures and cancellations	0.0	0.0	(0.2)	(3.5)
Expense recognized	(3.2)	(3.8)	(9.7)	(13.2)
Unamortized grant date fair value, ending balance	$ 15.8	$ 19.7	$ 15.8	$ 19.7
White Mountains | Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)	365	250
White Mountains | Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)		250
White Mountains | Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)	290	550		16,735
Sirius Group | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)		2,195		2,195

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

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