Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
White Mountains | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	53,815		70,620	66,470	70,620	70,675
Issued (in shares)	0			4,150	17,785	25,365
Vested (in shares)	(260)			0	(28,846)	(24,620)
Assumed forfeitures and cancellations	0			0	(6,004)	(800)
Ending of period (in shares)	53,555	53,815		70,620	53,555	70,620
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 23.5		$ 19.7	$ 24.2	$ 19.7	$ 15.7
Issued	0.0			3.4	16.7	19.7
Assumed forfeitures and cancellations	0.0			0.0	(3.5)	0.2
Expense recognized	(3.8)			(3.8)	(13.2)	(11.8)
Unamortized grant date fair value, ending balance	$ 19.7	$ 23.5		$ 23.8	$ 19.7	$ 23.8
White Mountains | Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)		250				750
White Mountains | Restricted Share Cliff Vesting in January 2019
Target Restricted Share Activity
Issued (in shares)		250				24,615
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