Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
White Mountains | Restricted Share Cliff Vesting in January 2021
Target Restricted Share Activity
Issued (in shares)	13,450
White Mountains | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	41,510	60,140	53,755	70,620
Issued (in shares)	0	1,050	14,105	17,785
Vested (in shares)	0	(6,571)	(25,381)	(28,586)
Assumed forfeitures and cancellations	0	(804)	(969)	(6,004)
Ending of period (in shares)	41,510	53,815	41,510	53,815
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 22.2	$ 29.1	$ 14.3	$ 19.7
Issued	0.0	0.9	11.4	16.7
Assumed forfeitures and cancellations	0.0	(0.7)	(0.2)	(3.5)
Expense recognized	(3.2)	(5.8)	(6.5)	(9.4)
Unamortized grant date fair value, ending balance	$ 19.0	$ 23.5	$ 19.0	$ 23.5
White Mountains | Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)	365	250
White Mountains | Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)		250
White Mountains | Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)	290	550		16,735
Sirius Group | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)		2,195		2,195