Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) $ in Millions, ¥ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 JPY (¥)
Changes in derivative instruments
Collective account values of underlying variable annuities as a percentage of the guarantee value					113.00%	113.00%
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Notional amount					$ 400.0	¥ 50.7
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			$ 0.0
Changes in derivative instruments
Beginning of period		$ 35.7	20.1	$ 56.4
Purchases		0.0		0.0
Realized and unrealized gains (losses)	$ 0.0	1.5	(2.0)	(6.3)
Transfers in		0.0	0.0	0.0
Sales/settlements		(0.2)	(18.1)	(13.1)
End of period		37.0		37.0
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	$ 0.0	0.0	(0.3)	0.0
Collective account values of underlying variable annuities as a percentage of the guarantee value					109.00%	109.00%
White Mountains Life Re | Level 2 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0.0
Changes in derivative instruments
Beginning of period		32.5	16.5	33.8
Purchases		0.0		0.0
Realized and unrealized gains (losses)		3.2	(0.7)	(2.7)
Transfers in		0.0	0.0	0.0
Sales/settlements		(7.8)	(15.8)	(3.2)
End of period		27.9		27.9
White Mountains Life Re | Level 1 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0.0
Changes in derivative instruments
Beginning of period		(5.1)	0.9	3.7
Purchases		0.0		0.0
Realized and unrealized gains (losses)		(5.1)	(4.2)	1.9
Transfers in		0.0	0.0	0.0
Sales/settlements		8.1	3.3	(7.7)
End of period		(2.1)		(2.1)
Variable Annuity | White Mountains Life Re | Level 3 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0.0
Changes in derivative instruments
Beginning balance		(1.8)	(0.3)	(0.7)
Realized and unrealized gains (losses)		(2.8)	(0.3)	(1.7)
Transfers in		0.0	0.0	0.0
Sales/settlements		0.0	0.0	0.0
Ending balance		1.0		1.0
Purchases		0.0		0.0
Derivative [Member] | White Mountains Life Re | Level 3 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases			0.0
Changes in derivative instruments
Beginning of period		8.3	2.7	18.9
Purchases		0.0		0.0
Realized and unrealized gains (losses)		3.4	2.9	(5.5)
Transfers in		0.0	0.0	0.0
Sales/settlements		(0.5)	$ (5.6)	(2.2)
End of period		$ 11.2		$ 11.2