Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
White Mountains | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	60,140	70,620	66,470	70,675	70,620	70,675
Issued (in shares)	1,050		0		17,785	21,215
Vested (in shares)	(6,571)		0		(28,586)	(24,620)
Assumed forfeitures and cancellations	(804)		0		(6,004)	(800)
Ending of period (in shares)	53,815	60,140	66,470	66,470	53,815	66,470
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 29.1	$ 19.7	$ 29.2	$ 15.7	$ 19.7	$ 15.7
Issued	0.9		0.0		16.7	16.3
Assumed forfeitures and cancellations	(0.7)		0.0		(3.5)	0.2
Expense recognized	(5.8)		(5.0)		(9.4)	(8.0)
Unamortized grant date fair value, ending balance	$ 23.5	$ 29.1	$ 24.2	$ 29.2	$ 23.5	$ 24.2
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 2019
Target Restricted Share Activity
Issued (in shares)	250			21,215
White Mountains | Restricted Share Cliff Vesting in January 2020
Target Restricted Share Activity
Issued (in shares)	550	16,735
Sirius Group | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)	2,195		5,235		2,195	5,235