Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended	3 Months Ended
	Dec. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Accrued Expense
Accrued expense at the end of the period	184.6	100.5
Performance Share | Performance Cycle 2011 to 2013
Accrued Expense
Vested (in shares)	3,570
Performance Share | W T M Incentive Plan | Performance Cycle 2011 to 2013
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 2010-2012
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)	119,220	123,549	119,220
Shares paid or expired (in shares)		(37,977)	(37,130)
New grants (in shares)		30,640	45,660
Assumed forfeitures and cancellations		184	(213)
Ending of period (in shares)		116,396	127,537
Accrued Expense
Accrued expense at the beginning of period	60.2	48.1	60.2
Payments and deferrals		(27.3)	(26.7)
Assumed forfeitures and cancellations		0.6	0.4
Expense recognized		6.2	8.4
Accrued expense at the end of the period		27.6	42.3
Vesting Period		3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested		33.1