Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2020
Target Performance Share activity
Issued (in shares)	17,485
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2019
Target Performance Share activity
Issued (in shares)	250	24,615
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2018
Target Performance Share activity
Issued (in shares)	250	750	23,640
White Mountains | W T M Incentive Plan | Restricted Stock
Target Performance Share activity
Beginning of period	70,620	70,675	83,314
Issued (in shares)	17,985	25,365	23,640
Vested (in shares)	(28,846)	(24,620)	(36,279)
Assumed forfeitures and cancellations (in shares)	(6,004)	(800)	0
End of period	53,755	70,620	70,675
Unamortized grant date fair value
Beginning balance	$ 19.7	$ 15.7	$ 14.3
Issued	16.3	20.2	15.7
Assumed forfeitures and cancellations	(3.5)	(0.3)	0.0
Expense recognized	(18.2)	(15.9)	(14.3)
Ending balance	$ 14.3	$ 19.7	$ 15.7
Sirius Group | W T M Incentive Plan | Restricted Stock
Target Performance Share activity
Beginning of period	5,235	9,205
End of period	2,195	5,235	9,205