Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 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)	40,602	40,616	40,616	50,515
Shares paid or expired (in shares)	0	0	(13,715)	(23,186)
New grants (in shares)	1,900	0	15,600	14,105
Assumed forfeitures and cancellations	(29)	0	(28)	(818)
Ending of period (in shares)	42,473	40,616	42,473	40,616
Accrued Expense
Accrued expense at the beginning of period	$ 23.1	$ 21.0	$ 31.7	$ 45.8
Payments and deferrals	0.0	0.0	(18.1)	(28.4)
Assumed forfeitures and cancellations	(0.2)	(0.1)	0.0	0.2
Expense recognized	7.9	6.3	17.2	9.6
Accrued expense at the end of the period	30.8	$ 27.2	30.8	$ 27.2
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 25.1		$ 25.1