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Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
White Mountains | Restricted Share Cliff Vesting in January 2020 [Member]
Target Restricted Share Activity
Issued (in shares)	16,735
White Mountains | Restricted Share Cliff Vesting in January 2019 [Member]
Target Restricted Share Activity
Issued (in shares)		21,215
White Mountains | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	70,620	70,675
Issued (in shares)	16,735	21,215
Vested (in shares)	(22,015)	(24,620)
Assumed forfeitures and cancellations	(5,200)	(800)
Ending of period (in shares)	60,140	66,470
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 19.7	$ 15.7
Issued	15.8	16.8
Assumed forfeitures and cancellations	(2.8)	(0.3)
Expense recognized	(3.6)	(3.0)
Unamortized grant date fair value, ending balance	$ 29.1	$ 29.2
Sirius Group | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)	5,235	2,195

X

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
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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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- Definition

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.

+ References

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- Definition

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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