Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2021 [Member]
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)	290	17,485
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2019
Target Performance Share activity
Issued (in shares)	365	250	24,615
W T M Incentive and Phantom Plan \| Restricted Share Cliff Vesting in January 2018
Target Performance Share activity
Issued (in shares)		250	750
White Mountains \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	53,755	70,620	70,675
Issued (in shares)	14,105	17,985	25,365
Vested (in shares)	(25,381)	(28,846)	(24,620)
Assumed forfeitures and cancellations (in shares)	(969)	(6,004)	(800)
End of period	41,510	53,755	70,620
Unamortized grant date fair value
Beginning balance	$ 14.3	$ 19.7	$ 15.7
Issued	11.4	16.3	20.2
Assumed forfeitures and cancellations	(0.2)	(3.5)	(0.3)
Expense recognized	(13.0)	(18.2)	(15.9)
Ending balance	$ 12.5	$ 14.3	$ 19.7
Sirius Group \| W T M Incentive Plan \| Restricted Stock
Target Performance Share activity
Beginning of period	2,195	5,235
End of period	0	2,195	5,235

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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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 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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Number of share instruments newly issued under a share-based compensation plan.

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

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