Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended		12 Months Ended
	May 05, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax		$ 0.0	$ 6.8	$ 0.0	$ 14.1
Non-Qualified Options [Member]
Share-based compensation arrangement by share-based payment award
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value		$ 100.0		$ 100.0
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage				50.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number Exercised	5,000
Target Restricted Share Activity
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number		120,000		120,000		125,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price	$ 742					$ 742
White Mountains | W T M Incentive Plan | Restricted Share Cliff Vesting in January 2019 [Member]
Target Restricted Share Activity
Issued (in shares)				21,215
White Mountains | W T M Incentive Plan | Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)					23,640
White Mountains | W T M Incentive Plan | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)		66,470	71,125	70,675	83,314	83,314
Issued (in shares)		0	750	21,215	23,640
Vested (in shares)		0	0	(24,620)	(35,079)
Assumed forfeitures and cancellations		0	0	(800)	0
Ending of period (in shares)		66,470	71,875	66,470	71,875	70,675
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance		$ 29.2	$ 26.5	$ 15.7	$ 14.3	$ 14.3
Issued		0.0	0.5	16.3	15.7
Assumed forfeitures and cancellations		0.0	0.0	0.2	0.0
Expense recognized		(5.0)	(3.8)	(8.0)	(6.8)
Unamortized grant date fair value, ending balance		$ 24.2	$ 23.2	$ 24.2	$ 23.2	$ 15.7
Sirius Group | W T M Incentive Plan | Restricted Stock
Target Restricted Share Activity
Ending of period (in shares)		2,512	3,572	2,512	3,572
Unamortized grant date fair value
Expense recognized		$ 1.4