Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified			6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Performance Share W T M Incentive Plan Performance Cycle 2011 to 2013	Jun. 30, 2013 Performance Share W T M Incentive Plan Performance cycle 2010-2012	Jun. 30, 2014 White Mountains Performance Share W T M Incentive Plan	Jun. 30, 2013 White Mountains Performance Share W T M Incentive Plan	Jun. 30, 2014 White Mountains Performance Share W T M Incentive Plan	Jun. 30, 2013 White Mountains Performance Share W T M Incentive Plan	Jun. 30, 2014 White Mountains Performance Cycle 2011 to 2013 Performance Share
Target Performance Share Activity
Beginning of period (in shares)					127,537	118,976	119,220	119,357
Shares paid or expired (in shares)					0	0	(37,130)	(47,310)
New grants (in shares)					0	0	45,660	46,920
Assumed forfeitures and cancellations					0	0	(213)	9
Ending of period (in shares)					127,537	118,976	127,537	118,976
Accrued Expense
Accrued expense at the beginning of period	$ 157.9	$ 218.3			$ 42.3	$ 28.6	$ 60.2	$ 29.4
Payments and deferrals					0	0	(26.7)	(11.0)
Assumed forfeitures and cancellations					(0.2)	(0.1)	0.2	0
Expense recognized					8.4	6.4	16.8	16.5
Accrued expense at the end of the period	157.9	218.3			50.5	34.9	50.5	34.9
Vesting Period							3 years
Percentage of target payout - minimum actual			88.00%	33.00%
Percentage of target payout - maximum actual			131.50%	98.00%
Vested (in shares)									3,570
Additional compensation cost that would be recognized if all outstanding performance shares vested					$ 46.4		$ 46.4