Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 7.3
Accrued Expense
Accrued expense at the beginning of period	141.7
Accrued expense at the end of the period	$ 114.2		$ 141.7
Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015
Accrued Expense
Percentage of target payout - minimum actual	140.00%
Percentage of target payout - maximum actual	142.00%
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Accrued Expense
Percentage of target payout - minimum actual			88.00%
Percentage of target payout - maximum actual			131.50%
White Mountains | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Beginning of period (in shares)	96,211	113,198	113,198
Shares paid or expired (in shares)	0	(34,001)
New grants (in shares)	16,215	29,078
Assumed forfeitures and cancellations	1,186	123
Ending of period (in shares)	111,240	108,398	96,211
Accrued Expense
Accrued expense at the beginning of period	$ 58.2	$ 44.4	$ 44.4
Payments and deferrals	0.0	(24.9)
Assumed forfeitures and cancellations	0.6	0.5
Expense recognized	14.2	5.7
Accrued expense at the end of the period	$ 71.8	$ 25.7	$ 58.2
Vesting Period	3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 35.2
Sirius Group | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Ending of period (in shares)	7,998