Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011		Sep. 30, 2012
Target Performance Share activity
Assumed forfeitures and cancellations (in shares)					0
Accrued Expense
Accrued expense at the end of the period	$ 218.3		$ 159.0
W T M Incentive Plan
Target Performance Share activity
Beginning of period (in shares)					46,250
Assumed forfeitures and cancellations (in shares)					0
Ending of period (in shares)					72,000
Accrued Expense
Assumed forfeitures and cancellations					0
Expense recognized					10.7
Vested (in shares)					(1,500)	[1]
W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Options nonvested, number before forfeitures (in shares)	122,277
Incentive compensation payable before forfeitures	61.7
Target Performance Share activity
Ending of period (in shares)	119,220
Accrued Expense
Accrued expense at the end of the period	60.2
forfeitures	(3,057)
Assumed forfeitures in period amount	(1.5)
W T M Incentive and Phantom Plan [Member] | Performance Shares [Member]
Target Performance Share activity
Beginning of period (in shares)	119,357		150,064		163,184		119,357
Shares paid or expired (in shares)	(47,310)	[2]	(68,357)	[2]	(51,131)	[2]
Assumed forfeitures and cancellations (in shares)	(3)	[3]	(782)	[3]	(336)	[3]
New grants (in shares)	47,170		38,432		37,675
Ending of period (in shares)	119,220		119,357		150,064		119,357
Accrued Expense
Accrued expense at the beginning of period	29.4		66.1		29.4		29.4
Payments and deferrals	(11.0)	[2]	(58.3)	[2]	0	[2]
Assumed forfeitures and cancellations	(0.6)	[3]	0.6	[3]	(0.9)	[3]
Expense recognized	42.4		21.0		37.6
Accrued expense at the end of the period	60.2		29.4		66.1		29.4
Performance Cycle From 2011 to 2013 | W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	10.00%
Options nonvested, number before forfeitures (in shares)	37,130
Incentive compensation payable before forfeitures	28.2
Accrued Expense
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2013 to 2015 | W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Options nonvested, number before forfeitures (in shares)	47,170
Incentive compensation payable before forfeitures	12.1
Accrued Expense
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2012 to 2014 | W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share	8.00%
Options nonvested, number before forfeitures (in shares)	37,977
Incentive compensation payable before forfeitures	$ 21.4
Accrued Expense
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%
Performance Cycle From 2011 to 2013 | Minimum | W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	3.00%
Performance Cycle From 2011 to 2013 | Maximum | W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	17.00%
Performance Cycle From 2012 to 2014 | Minimum | W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Performance Cycle From 2012 to 2014 | Maximum | W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%
Performance Cycle From 2013 to 2015 | Minimum | W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	2.00%
Performance Cycle From 2013 to 2015 | Maximum | W T M Incentive Plan | Performance Shares [Member]
Share-based compensation arrangement by share-based payment award
Performance goal percentage for minimum payout	14.00%

[1] During 2012, the Compensation Committee accelerated the vesting date for an installment of 5,000 restricted shares from January 20, 2013 to December 31, 2012.

[2] WTM performance shares payments in 2013 for the 2010-2012 performance cycle ranged from 33% to 98% of target. WTM performance share payments in 2012 for the 2009-2011 performance cycle ranged from 147% to 155% of target. At December 31, 2012, White Mountains paid $9.9 as a partial payment for the 2010-2012 performance cycle. There were no payments made in 2011 for the 2008-2010 performance cycle; those performance shares did not meet the threshold performance goal and expired.

[3] Amounts include changes in assumed forfeitures, as required under GAAP.