Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - White Mountains - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	41,510	53,755
Issued (in shares)	13,700	14,105
Vested (in shares)	(13,715)	(25,381)
Assumed forfeitures and cancellations	0	(969)
Ending of period (in shares)	41,495	41,510
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 12.5	$ 14.3
Issued	12.8	11.4
Assumed forfeitures and cancellations	0.0	(0.2)
Expense recognized	(2.5)	(3.3)
Unamortized grant date fair value, ending balance	$ 22.8	$ 22.2
Restricted Share Cliff Vesting in January 2022
Target Restricted Share Activity
Issued (in shares)	13,700
Restricted Share Cliff Vesting in January 2021
Target Restricted Share Activity
Issued (in shares)		13,450
Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)		290
Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)		365

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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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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