Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - White Mountains - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	43,395	41,510
Issued (in shares)	14,055	13,700
Vested (in shares)	(14,345)	(13,715)
Assumed forfeitures and cancellations	0	0
Ending of period (in shares)	43,105	41,495
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 16.7	$ 12.5
Issued	15.1	12.8
Assumed forfeitures and cancellations	0.0	0.0
Expense recognized	(5.2)	(2.5)
Unamortized grant date fair value, ending balance	$ 26.6	$ 22.8
Restricted Share Cliff Vesting in January 2022
Unamortized grant date fair value
Target Performance Shares Outstanding	14,055
Restricted Share Cliff Vesting in January 2021
Unamortized grant date fair value
Target Performance Shares Outstanding		13,700