Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Range Of Performance Cycle From 2014-2016
Accrued Expense
Percentage of target payout - minimum actual					34.00%
Percentage of target payout - maximum actual					76.00%
Performance Cycle 2015 to 2017
Accrued Expense
Percentage of target payout - minimum actual			145.00%
Percentage of target payout - maximum actual			147.00%
White Mountains
Share-based compensation arrangement by share-based payment award
Vesting Period			3 years
Target Performance Share Activity
Beginning of period (in shares)	40,616	56,805	50,515	80,353	80,353
Shares paid or expired (in shares)	0	(671)	(23,186)	(30,838)
New grants (in shares)	0	1,050	14,105	17,510
Assumed forfeitures and cancellations	0	(6,609)	(818)	(16,450)
Ending of period (in shares)	40,616	50,575	40,616	50,575	50,515
Accrued Expense
Accrued expense at the beginning of period	$ 21.0	$ 24.0	$ 45.8	$ 42.4	$ 42.4
Payments and deferrals	0.0	(0.8)	(28.4)	(21.6)
Assumed forfeitures and cancellations	(0.1)	(3.3)	0.2	(9.0)
Expense recognized	6.3	11.3	9.6	19.4
Accrued expense at the end of the period	27.2	$ 31.2	27.2	$ 31.2	$ 45.8
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 23.4		$ 23.4
Sirius Group
Target Performance Share Activity
Ending of period (in shares)		2,195		2,195