Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accrued Expense
Accrued expense at the beginning of period		$ 95.7		$ 95.7
Accrued expense at the end of the period	$ 76.9			$ 76.9		$ 95.7
Common shares, issued shares	3,749,971			3,749,971		4,563,814
Performance Share | W T M Incentive Plan | 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 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%
Common shares, issued shares						5,000
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period				3 years
Target Performance Share Activity
Beginning of period (in shares)	50,575	80,353	73,297	80,353	93,654	93,654
Shares paid or expired (in shares)	0		0	(30,838)	(36,294)
New grants (in shares)	0		6,400	17,510	22,615
Assumed forfeitures and cancellations	(256)		(160)	(16,706)	(438)
Ending of period (in shares)	50,319		79,537	50,319	79,537	80,353
Accrued Expense
Accrued expense at the beginning of period	$ 31.2	$ 42.4	$ 32.5	$ 42.4	$ 57.7	$ 57.7
Payments and deferrals	0.0		0.0	(21.6)	(41.0)
Assumed forfeitures and cancellations	(0.4)		(0.1)	(9.4)	0.2
Expense recognized	7.0		5.0	26.4	20.5
Accrued expense at the end of the period	37.8		$ 37.4	37.8	$ 37.4	$ 42.4
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 28.7			$ 28.7
Sirius Group | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Ending of period (in shares)	2,195		7,315	2,195	7,315