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Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended			6 Months Ended	9 Months Ended
	May 05, 2016	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax		$ 0.0		$ 3.9		$ 0.0	$ 18.0
Non-Qualified Options [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value		$ 100.0				$ 100.0
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage						50.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number Exercised	5,000
Target Restricted Share Activity
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number		120,000				120,000		125,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price	$ 742							$ 742
White Mountains | W T M Incentive Plan | Restricted Share Cliff Vesting in January 2019 [Member]
Target Restricted Share Activity
Issued (in shares)		4,150				25,365
White Mountains | W T M Incentive Plan | Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)							23,640
White Mountains | W T M Incentive Plan | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)		66,470		71,875	83,314	70,675	83,314
Issued (in shares)		4,150				25,365	23,640
Vested (in shares)		0		(1,200)		(24,620)	(36,279)
Assumed forfeitures and cancellations		0		0		(800)	0
Ending of period (in shares)		70,620	66,470	70,675	71,875	70,620	70,675
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance		$ 24.2		$ 23.2	$ 14.3	$ 15.7	$ 14.3
Issued		3.4				19.7	15.7
Assumed forfeitures and cancellations		0.0		0.0		0.2	0.0
Expense recognized		(3.8)		(3.7)		(11.8)	(10.5)
Unamortized grant date fair value, ending balance		$ 23.8	$ 24.2	$ 19.5	$ 23.2	$ 23.8	$ 19.5
Sirius Group | W T M Incentive Plan | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)		2,512		3,572		2,512	3,572
Unamortized grant date fair value
Expense recognized			$ 1.4

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
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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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Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

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Number of options outstanding, including both vested and non-vested options.

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

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Percentage of vesting of share-based compensation awards.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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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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Number of non-qualified options exercised under an established share-based compensation plan.

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