Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Share-based compensation arrangement by share-based payment award
Common shares, issued shares	4,572,792		4,563,814
Accrued Expense
Accrued expense at the beginning of period	$ 140.0
Accrued expense at the end of the period	$ 83.2		$ 140.0
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2014-2016 [Member]
Accrued Expense
Percentage of target payout - minimum actual	34.00%
Percentage of target payout - maximum actual	76.00%
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
Accrued Expense
Percentage of target payout - minimum actual			140.00%
Percentage of target payout - maximum actual			142.00%
White Mountains | Restricted Share Cliff Vesting in January 2020 [Member] | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Issued (in shares)	16,735
White Mountains | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Beginning of period (in shares)	80,353	93,654	93,654
Shares paid or expired (in shares)	(30,167)	0
New grants (in shares)	16,460	16,215
Assumed forfeitures and cancellations	(9,841)	(1,186)
Ending of period (in shares)	56,805	108,683	80,353
Accrued Expense
Accrued expense at the beginning of period	$ 42.4	$ 57.7	$ 57.7
Payments and deferrals	(20.8)	0.0
Assumed forfeitures and cancellations	(5.7)	(0.6)
Expense recognized	8.1	14.2
Accrued expense at the end of the period	$ 24.0	$ 71.3	$ 42.4
Vesting Period	3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 30.5
Sirius Group | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Ending of period (in shares)	2,195	10,826