Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - White Mountains - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	43,105	41,495	43,395	41,510
Issued (in shares)	0	1,900	14,055	15,600
Vested (in shares)	0	0	(14,345)	(13,715)
Assumed forfeitures and cancellations	0	0	0	0
Ending of period (in shares)	43,105	43,395	43,105	43,395
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 26.6	$ 22.8	$ 16.7	$ 12.5
Issued	0.0	1.7	15.1	14.5
Assumed forfeitures and cancellations	0.0	0.0	0.0	0.0
Expense recognized	(3.6)	(2.6)	(8.8)	(5.1)
Unamortized grant date fair value, ending balance	$ 23.0	$ 21.9	$ 23.0	$ 21.9
Restricted Share Cliff Vesting in January 2022
Unamortized grant date fair value
Target Performance Shares Outstanding	14,055		14,055
Restricted Share Cliff Vesting in January 2022 [Member]
Unamortized grant date fair value
New grants (in shares)		1,900		15,600