Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2015
Variable Annuity Reinsurance [Line Items]
Notional amount	$ 299.9
Changes in derivative instruments
Collective account values of underlying variable annuities as a percentage of the guarantee value			113.00%
White Mountains Life Re
Changes in derivative instruments
Beginning of period	$ 0.0	$ 20.1
Purchases		0.0
Realized and unrealized gains (losses)		(1.7)
Transfers in		0.0
Sales/settlements		(5.3)
End of period		13.1
White Mountains Life Re | Level 2 Inputs
Changes in derivative instruments
Beginning of period		16.5
Purchases		0.0
Realized and unrealized gains (losses)		1.1
Transfers in		0.0
Sales/settlements		(7.3)
End of period		10.3
White Mountains Life Re | Level 1 Inputs
Changes in derivative instruments
Beginning of period		0.9
Purchases		0.0
Realized and unrealized gains (losses)		(4.0)
Transfers in		0.0
Sales/settlements		3.3
End of period		0.2
Variable Annuity | White Mountains Life Re | Level 3 Inputs
Changes in derivative instruments
Beginning balance		(0.3)
Realized and unrealized gains (losses)		(0.4)
Transfers in		0.0
Sales/settlements		0.0
Ending balance		0.1
Purchases		0.0
Derivative [Member] | White Mountains Life Re | Level 3 Inputs
Changes in derivative instruments
Beginning of period		2.7
Purchases		0.0
Realized and unrealized gains (losses)		1.2
Transfers in		0.0
Sales/settlements		(1.3)
End of period		$ 2.6