Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2020 USD ($) shares	Mar. 31, 2020 USD ($) shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2020 USD ($) multiplier shares	Jun. 30, 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	42,458	42,473	40,602	42,473	40,616
Shares paid or expired (in shares) | shares	0		0	(14,070)	(13,715)
New grants (in shares) | shares	0		1,900	14,055	15,600
Assumed forfeitures and cancellations | shares	0		(29)	0	(28)
Ending of period (in shares) | shares	42,458	42,458	42,473	42,458	42,473
Accrued Expense
Accrued expense at the beginning of period	$ 9.8	$ 43.7	$ 23.1	$ 43.7	$ 31.7
Payments and deferrals	0.0		0.0	(27.7)	(18.1)
Assumed forfeitures and cancellations	0.0		(0.2)	0.4	0.0
Expense recognized	6.0		7.9	(0.6)	17.2
Accrued expense at the end of the period	15.8	$ 9.8	$ 30.8	15.8	$ 30.8
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 17.3			$ 17.3