Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified			9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Performance Share W T M Incentive Plan Performance Cycle 2011 to 2013	Sep. 30, 2013 Performance Share W T M Incentive Plan Performance cycle 2010-2012	Sep. 30, 2014 White Mountains Performance Share W T M Incentive Plan	Sep. 30, 2013 White Mountains Performance Share W T M Incentive Plan	Sep. 30, 2014 White Mountains Performance Share W T M Incentive Plan	Sep. 30, 2013 White Mountains Performance Share W T M Incentive Plan	Sep. 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	250	45,660	47,170
Assumed forfeitures and cancellations					0	(6)	(213)	3
Ending of period (in shares)					127,537	119,220	127,537	119,220
Accrued Expense
Accrued expense at the beginning of period	$ 187.5	$ 218.3			$ 50.5	$ 34.9	$ 60.2	$ 29.4
Payments and deferrals					0	0	(26.7)	(11.0)
Assumed forfeitures and cancellations					(0.2)	(0.2)	0	(0.2)
Expense recognized					6.8	7.1	23.6	23.6
Accrued expense at the end of the period	187.5	218.3			57.1	41.8	57.1	41.8
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					$ 38.6		$ 38.6