Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - White Mountains - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	43,395	41,510	41,510	53,755
Issued (in shares)	0	0	15,600	14,105
Vested (in shares)	0	0	(13,715)	(25,381)
Assumed forfeitures and cancellations	0	0	0	(969)
Ending of period (in shares)	43,395	41,510	43,395	41,510
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 21.9	$ 19.0	$ 12.5	$ 14.3
Issued	0.0	0.0	14.5	11.4
Assumed forfeitures and cancellations	0.0	0.0	0.0	(0.2)
Expense recognized	(2.6)	(3.2)	(7.7)	(9.7)
Unamortized grant date fair value, ending balance	$ 19.3	$ 15.8	$ 19.3	$ 15.8
Restricted Share Cliff Vesting in January 2022
Target Restricted Share Activity
Issued (in shares)			15,600
Restricted Share Cliff Vesting in January 2021
Target Restricted Share Activity
Issued (in shares)				13,450
Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)				290
Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)				365