Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Accrued Expense
Accrued expense at the beginning of period			$ 108.1
Accrued expense at the end of the period	$ 121.0		$ 121.0		$ 108.1
Performance Share | Range Of Performance Cycle From 2012 To 2014 [Member]
Accrued Expense
Vested (in shares)					1,425
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Accrued Expense
Percentage of target payout - minimum actual			91.00%
Percentage of target payout - maximum actual			145.50%
Performance Share | W T M Incentive Plan | Performance Cycle 2011 to 2013
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)	109,129	116,259	113,198	108,605	108,605
Shares paid or expired (in shares)	(8,958)	0	(42,958)	(33,730)
New grants (in shares)	0	0	29,828	41,580
Assumed forfeitures and cancellations	(3,960)	0	(3,857)	(196)
Ending of period (in shares)	96,211	116,259	96,211	116,259	113,198
Accrued Expense
Accrued expense at the beginning of period	$ 28.8	$ 46.3	$ 44.4	$ 54.9	$ 54.9
Payments and deferrals	(5.5)	0.0	(30.7)	(24.1)
Assumed forfeitures and cancellations	(0.8)	(0.2)	(0.3)	0.0
Expense recognized	30.0	6.3	39.1	21.6
Accrued expense at the end of the period	52.5	$ 52.4	$ 52.5	$ 52.4	$ 44.4
Vesting Period			3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 30.2		$ 30.2
Sirius Group | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Ending of period (in shares)	7,997	11,278	7,997	11,278