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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) - Performance Shares $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) multiplier shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares
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	40,616	50,515	80,353
Shares paid or expired	(13,715)	(23,186)	(30,838)
New grants	15,600	14,105	17,710
Forfeitures	(28)	(818)	(16,710)
End of period	42,473	40,616	50,515
Unamortized grant date fair value
Beginning of period | $	$ 31.7	$ 45.8	$ 42.4
Shares paid or expired | $	(18.1)	(28.4)	(21.9)
Forfeitures | $	(0.1)	0.1	(9.3)
Expense recognized | $	30.2	14.2	34.6
End of period | $	43.7	$ 31.7	$ 45.8
Unamortized grant date fair value | $	$ 17.2
W T M Incentive Plan | Sirius Group
Target Performance Share activity
Beginning of period		2,195
End of period			2,195
W T M Incentive and Phantom Plan | Range Of Performance Cycle From 2016-2018
Unamortized grant date fair value
Performance goal percentage for minimum payout	139.00%
Payout for maximum growth target percentage	166.00%
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%

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- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The total reduction to unamortized compensation cost for performance shares forfeited during the period.

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