Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Performance Cycle 2016 to 2018
Accrued Expense
Percentage of target payout - minimum actual	139.00%
Percentage of target payout - maximum actual	166.00%
Performance Cycle 2015 to 2017
Accrued Expense
Percentage of target payout - minimum actual	145.00%
Percentage of target payout - maximum actual	147.00%
White Mountains
Share-based compensation arrangement by share-based payment award
Vesting Period	3 years
Target Performance Share Activity
Beginning of period (in shares)	40,616	50,515
Shares paid or expired (in shares)	(13,715)	(23,186)
New grants (in shares)	13,700	14,105
Assumed forfeitures and cancellations	1	(818)
Ending of period (in shares)	40,602	40,616
Accrued Expense
Accrued expense at the beginning of period	$ 31.7	$ 45.8
Payments and deferrals	(18.1)	(28.4)
Assumed forfeitures and cancellations	0.2	0.3
Expense recognized	9.3	3.3
Accrued expense at the end of the period	23.1	$ 21.0
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 24.4