Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) - White Mountains Life Re - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Changes in derivative instruments
Beginning of period		$ 0.0	$ 20.1
Purchases			0.0
Realized and unrealized gains (losses)	$ 0.0		(2.0)
Transfers in			0.0
Sales/settlements			(18.1)
End of period	0.0		0.0
Level 2 Inputs
Changes in derivative instruments
Beginning of period			16.5
Purchases			0.0
Realized and unrealized gains (losses)			(0.7)
Transfers in			0.0
Sales/settlements			(15.8)
End of period	0.0		0.0
Level 1 Inputs
Changes in derivative instruments
Beginning of period			0.9
Purchases			0.0
Realized and unrealized gains (losses)			(4.2)
Transfers in			0.0
Sales/settlements			3.3
End of period	0.0		0.0
Variable Annuity | Level 3 Inputs
Changes in derivative instruments
Beginning balance			(0.3)
Realized and unrealized gains (losses)			(0.3)
Transfers in			0.0
Sales/settlements			0.0
Ending balance	0.0		0.0
Purchases			0.0
Derivative | Level 3 Inputs
Changes in derivative instruments
Beginning of period			2.7
Purchases			0.0
Realized and unrealized gains (losses)			2.9
Transfers in			0.0
Sales/settlements			(5.6)
End of period	$ 0.0		$ 0.0