Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) multiplier shares	Mar. 31, 2019 USD ($) shares
W T M Incentive Plan | Performance Cycle 2016 to 2018
Accrued Expense
Percentage of target payout - minimum actual		139.00%
Percentage of target payout - maximum actual		166.00%
W T M Incentive Plan | Performance Cycle 2018 to 2020
Accrued Expense
Percentage of target payout - minimum actual	174.00%
Percentage of target payout - maximum actual	180.00%
White Mountains
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Multiplier for Determining Payout, Low End of Range | multiplier	0
Accrued Expense
Share-based Compensation Arrangement by Share-based Payment Award, Multiplier for Determining Payout, High End of Range | multiplier	2
Share-based Compensation Arrangement by Share-based Payment Award, Performance Cycle Period	3 years
White Mountains | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period	3 years
Target Performance Share Activity
Beginning of period (in shares) | shares	42,473	40,616
Shares paid or expired (in shares) | shares	(14,070)	(13,715)
New grants (in shares) | shares	14,055	13,700
Assumed forfeitures and cancellations | shares	0	1
Ending of period (in shares) | shares	42,458	40,602
Accrued Expense
Accrued expense at the beginning of period	$ 43.7	$ 31.7
Payments and deferrals	(27.7)	(18.1)
Assumed forfeitures and cancellations	0.4	0.2
Expense recognized	(6.6)	9.3
Accrued expense at the end of the period	9.8	$ 23.1
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 18.5