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Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2022
Target Performance Share activity
Issued (in shares)	15,600
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2021
Target Performance Share activity
Issued (in shares)		13,450
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2020
Target Performance Share activity
Issued (in shares)			17,485
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2019
Target Performance Share activity
Issued (in shares)		290	250
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2018
Target Performance Share activity
Issued (in shares)		365	250
White Mountains | W T M Incentive Plan | Restricted Stock
Target Performance Share activity
Beginning of period	41,510	53,755	70,620
Issued (in shares)	15,600	14,105	17,985
Vested (in shares)	(13,715)	(25,381)	(28,846)
Assumed forfeitures and cancellations (in shares)	0	(969)	(6,004)
End of period	43,395	41,510	53,755
Unamortized grant date fair value
Beginning balance	$ 12.5	$ 14.3	$ 19.7
Issued	14.5	11.4	16.3
Assumed forfeitures and cancellations	0.0	(0.2)	(3.5)
Expense recognized	(10.3)	(13.0)	(18.2)
Ending balance	$ 16.7	$ 12.5	$ 14.3
Sirius Group | W T M Incentive Plan | Restricted Stock
Target Performance Share activity
Beginning of period		2,195
End of period			2,195

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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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