Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Common shares, issued shares	4,963,921		4,963,921		5,623,735
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 6.8	$ 0.0	$ 14.1
Accrued Expense
Accrued expense at the beginning of period			140.7
Accrued expense at the end of the period	$ 106.3		$ 106.3		$ 140.7
Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Common shares, issued shares	5,000		5,000
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)	111,387	108,545	96,358	113,295	113,295
Shares paid or expired (in shares)	(36,294)	0	(36,294)	(34,001)
New grants (in shares)	0	750	16,215	29,878
Assumed forfeitures and cancellations	908	(19)	(278)	104
Ending of period (in shares)	76,001	109,276	76,001	109,276	96,358
Accrued Expense
Accrued expense at the beginning of period	$ 71.3	$ 25.7	$ 57.7	$ 44.4	$ 44.4
Payments and deferrals	(41.3)	0.0	(41.3)	(24.9)
Assumed forfeitures and cancellations	0.9	0.0	0.3	0.5
Expense recognized	1.3	3.1	15.5	8.8
Accrued expense at the end of the period	32.2	$ 28.8	$ 32.2	$ 28.8	$ 57.7
Vesting Period			3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 27.2		$ 27.2
Sirius Group | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Ending of period (in shares)	4,428	7,851	4,428	7,851