Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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	50,575	50,515	80,353	80,353
Shares paid or expired (in shares)	0	0	(23,186)	(30,838)
New grants (in shares)	0	0	14,105	17,510
Assumed forfeitures and cancellations	0	(256)	(818)	(16,706)
Ending of period (in shares)	40,616	50,319	40,616	50,319	50,515
Accrued Expense
Accrued expense at the beginning of period	$ 27.2	$ 31.2	$ 45.8	$ 42.4	$ 42.4
Payments and deferrals	0.0	0.0	(28.4)	(21.6)
Assumed forfeitures and cancellations	(0.1)	(0.4)	0.1	(9.4)
Expense recognized	7.8	7.0	17.4	26.4
Accrued expense at the end of the period	34.9	$ 37.8	34.9	$ 37.8	$ 45.8
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 20.4		$ 20.4
Sirius Group
Target Performance Share Activity
Ending of period (in shares)		2,195		2,195