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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 1

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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- Definition

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

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-Name Accounting Standards Codification

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- Definition

The total reduction to unamortized compensation cost for performance shares forfeited during the period.

+ References

No definition available.

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- Definition

The total fair value (on the date of issue) for restricted share awards issued during the period.

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No definition available.

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- Definition

Number of restricted shares installments.

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Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			3 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2014
Restricted Share Cliff Vesting in January 2016 [Member]
Target Performance Share activity
Issued (in shares)	25,720
Restricted Share Cliff Vesting in January 2015 [Member]
Target Performance Share activity
Issued (in shares)	27,960
Restricted Share Vesting Beginning in February 2014 [Member]
Target Performance Share activity
Issued (in shares)	3,000
Unamortized grant date fair value
Number of restricted shares installments	2
Restricted Share Cliff Vesting in July 2015 [Member]
Target Performance Share activity
Issued (in shares)	1,200
W T M Incentive and Phantom Plan [Member] \| Restricted Share Cliff Vesting in January 2017 [Member]
Target Performance Share activity
Issued (in shares)		23,440
Restricted Share Vested in December 2012 [Member] \| Restricted Stock
Unamortized grant date fair value
Number of restricted shares installments			5,000
White Mountains \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	69,910	94,130	72,000
Issued (in shares)	25,720	23,440	32,160
Vested (in shares)	(1,500)	(33,205)	(32,945)
Assumed forfeitures and cancellations (in shares)	0	(1,051)	(1,305)
Ending December 31,	94,130	83,314	69,910	83,314
Unamortized grant date fair value
Beginning balance	16.8	17.0	$ 13.3
Issued	14.4	13.1	15.7
Assumed forfeitures and cancellations	0	(0.5)	(0.2)
Expense recognized	14.2	15.3	(12.0)
Ending balance	17.0	14.3	16.8	14.3
White Mountains \| W T M Incentive Plan \| Performance Shares
Target Performance Share activity
Beginning of period	119,357	119,220	150,064
Assumed forfeitures and cancellations (in shares)	3	(4,201)	(782)
Ending December 31,	119,220	123,549	119,357	123,549
Unamortized grant date fair value
Assumed forfeitures and cancellations	(0.6)	(0.7)	0.6
Expense recognized	(42.4)	(15.3)	(21.0)
Ending balance		16.9		16.9
Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive and Phantom Plan [Member] \| Restricted Stock
Target Performance Share activity
Issued (in shares)				3,040
Assumed forfeitures and cancellations (in shares)				(3,040)
Range Of Performance Cycle From 2014-2016 [Member] \| W T M Incentive and Phantom Plan [Member] \| Performance Shares
Target Performance Share activity
Assumed forfeitures and cancellations (in shares)				(3,040)