Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Accrued Expense
Accrued expense at the beginning of period			$ 184.6
Accrued expense at the end of the period	$ 124.9		$ 124.9		$ 184.6
Performance Share | Range Of Performance Cycle From 2012 To 2014 [Member]
Accrued Expense
Vested (in shares)					3,570
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Accrued Expense
Percentage of target payout - minimum actual			91.00%
Percentage of target payout - maximum actual			145.50%
Performance Share | W T M Incentive Plan | Performance Cycle 2011 to 2013
Accrued Expense
Percentage of target payout - minimum actual					88.00%
Percentage of target payout - maximum actual					131.50%
White Mountains | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Beginning of period (in shares)	116,396	127,537	123,549	119,220	119,220
Shares paid or expired (in shares)	0	0	(37,977)	(37,130)
New grants (in shares)	750	0	31,390	45,660
Assumed forfeitures and cancellations	(19)	0	165	(213)
Ending of period (in shares)	117,127	127,537	117,127	127,537	123,549
Accrued Expense
Accrued expense at the beginning of period	$ 27.6	$ 42.3	$ 48.1	$ 60.2	$ 60.2
Payments and deferrals	0.0	0.0	(27.3)	(26.7)
Assumed forfeitures and cancellations	(0.1)	(0.2)	0.5	0.2
Expense recognized	3.6	8.4	9.8	16.8
Accrued expense at the end of the period	31.1	$ 50.5	$ 31.1	$ 50.5	$ 48.1
Vesting Period			3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 25.9		$ 25.9