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Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 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
Common shares, issued shares	4,578,698			4,578,698		5,623,735
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 3.9		$ 0.0	$ 18.0
Accrued Expense
Accrued expense at the beginning of period				140.7
Accrued expense at the end of the period	$ 124.2			$ 124.2		$ 140.7
Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Common shares, issued shares	5,000			5,000
Accrued Expense
Percentage of target payout - minimum actual				140.00%
Percentage of target payout - maximum actual				142.00%
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Accrued Expense
Percentage of target payout - minimum actual						88.00%
Percentage of target payout - maximum actual						131.50%
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2015 to 2017 [Member]
Target Performance Share Activity
New grants (in shares)	3,000
Performance Share | W T M Incentive Plan | Performance Cycle 2016 to 2018
Target Performance Share Activity
New grants (in shares)	3,400
White Mountains | Restricted Share Cliff Vesting in January 2019 [Member] | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Issued (in shares)	4,150			25,365
White Mountains | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Beginning of period (in shares)	73,297	106,572	111,257	93,654	111,257	111,257
Shares paid or expired (in shares)		(8,958)		(36,294)	(42,959)
New grants (in shares)	6,400			22,615	29,195
Assumed forfeitures and cancellations	(160)	(3,960)		(438)	(3,839)
Ending of period (in shares)	79,537	93,654	106,572	79,537	93,654	93,654
Accrued Expense
Accrued expense at the beginning of period	$ 32.5	$ 28.8	$ 44.4	$ 57.7	$ 44.4	$ 44.4
Payments and deferrals		(5.5)		(41.0)	(30.4)
Assumed forfeitures and cancellations	(0.1)	(0.8)		(0.2)	(0.3)
Expense recognized	5.0	30.0		20.5	38.8
Accrued expense at the end of the period	37.4	$ 52.5	$ 28.8	$ 37.4	$ 52.5	$ 57.7
Vesting Period				3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 26.9			$ 26.9
Sirius Group | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Ending of period (in shares)	7,315	10,825		7,315	10,825

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- 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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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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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 grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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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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Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements.

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The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria.

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The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria.

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