Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) - Performance Shares $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) multiplier shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares
Range Of Performance Cycle From 2013 to 2015
Unamortized grant date fair value
Vested (in shares)	5,000
White Mountains
Share-based compensation arrangement by share-based payment award
Multiplier for determining payout, low end of range | multiplier	0
Multiplier for determining payout, high end of range | multiplier	2
Performance cycle period	3 years
W T M Incentive Plan | White Mountains
Target Performance Share activity
Beginning of period	50,515	80,353	93,654
Shares paid or expired	(23,186)	(30,838)	(36,294)
New grants	14,105	17,710	22,615
Forfeitures	(818)	(16,710)	378
End of period	40,616	50,515	80,353
Unamortized grant date fair value
Beginning of period | $	$ 45.8	$ 42.4	$ 57.7
Shares paid or expired | $	(28.4)	(21.9)	41.0
Forfeitures | $	0.1	(9.3)	0.5
Expense recognized | $	14.2	34.6	25.2
End of period | $	31.7	$ 45.8	$ 42.4
Unamortized grant date fair value | $	$ 13.8
W T M Incentive Plan | Sirius Group
Target Performance Share activity
Beginning of period	2,195	7,315
End of period	0	2,195	7,315
W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2015 to 2017
Unamortized grant date fair value
Performance goal percentage for minimum payout	145.00%
Payout for maximum growth target percentage	147.00%
W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2014 to 2016
Unamortized grant date fair value
Performance goal percentage for minimum payout		34.00%
Payout for maximum growth target percentage		76.00%
W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2013 to 2015
Unamortized grant date fair value
Performance goal percentage for minimum payout			140.00%
Payout for maximum growth target percentage			142.00%