Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 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)	50,515	80,353	80,353
Shares paid or expired (in shares)	(23,186)	(30,167)
New grants (in shares)	14,105	16,460
Assumed forfeitures and cancellations	(818)	(9,841)
Ending of period (in shares)	40,616	56,805	50,515
Accrued Expense
Accrued expense at the beginning of period	$ 45.8	$ 42.4	$ 42.4
Payments and deferrals	(28.4)	(20.8)
Assumed forfeitures and cancellations	0.3	(5.7)
Expense recognized	3.3	8.1
Accrued expense at the end of the period	21.0	$ 24.0	$ 45.8
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 26.1
Sirius Group
Target Performance Share Activity
Ending of period (in shares)		2,195