Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) - White Mountains Life Re - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Changes in derivative instruments
Beginning of period	$ 13.1	$ 0.0	$ 20.1
Purchases	0.0		0.0
Realized and unrealized gains (losses)	(0.3)		(2.0)
Transfers in	0.0		0.0
Sales/settlements	(12.8)		(18.1)
End of period	0.0		0.0
Level 2 Inputs
Changes in derivative instruments
Beginning of period	10.3		16.5
Purchases	0.0		0.0
Realized and unrealized gains (losses)	(1.8)		(0.7)
Transfers in	0.0		0.0
Sales/settlements	(8.5)		(15.8)
End of period	0.0		0.0
Level 1 Inputs
Changes in derivative instruments
Beginning of period	0.2		0.9
Purchases	0.0		0.0
Realized and unrealized gains (losses)	(0.2)		(4.2)
Transfers in	0.0		0.0
Sales/settlements			3.3
End of period	0.0		0.0
Variable Annuity | Level 3 Inputs
Changes in derivative instruments
Beginning balance	0.1		(0.3)
Realized and unrealized gains (losses)	0.1		(0.3)
Transfers in	0.0		0.0
Sales/settlements	0.0		0.0
Ending balance	0.0		0.0
Purchases	0.0		0.0
Derivative | Level 3 Inputs
Changes in derivative instruments
Beginning of period	2.6		2.7
Purchases	0.0		0.0
Realized and unrealized gains (losses)	1.7		2.9
Transfers in	0.0		0.0
Sales/settlements	(4.3)		(5.6)
End of period	$ 0.0		$ 0.0