Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
White Mountains | Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)			22,078
White Mountains | Restricted Share Cliff Vesting in January 2017
Target Restricted Share Activity
Issued (in shares)			20,400
White Mountains | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	68,303	79,011	80,050	90,341	90,341
Issued (in shares)	0	0	22,078	20,400
Vested (in shares)	(1,200)	0	(35,025)	(31,730)
Assumed forfeitures and cancellations	0	0	0	0
Ending of period (in shares)	67,103	79,011	67,103	79,011	80,050
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 21.9	$ 20.5	$ 13.7	$ 16.4	$ 16.4
Issued	0.0	0.0	14.7	11.8
Assumed forfeitures and cancellations	0.0	0.0	0.0	0.0
Expense recognized	(3.5)	(3.2)	(10.0)	(10.9)
Unamortized grant date fair value, ending balance	$ 18.4	$ 17.3	$ 18.4	$ 17.3	$ 13.7
Sirius Group | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)	3,571	2,314	3,571	2,314