Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) $ in Millions, ¥ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 JPY (¥)	Dec. 31, 2014 USD ($)	Dec. 31, 2014 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	$ 600.0		$ 600.0		¥ 76.3	$ 1,100.0	¥ 134.2
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Changes in derivative instruments
Beginning of period	35.7	$ 74.8	56.4	$ 69.2
Purchases		0.0		0.0
Realized and unrealized gains (losses)	1.5	(23.8)	(6.3)	(56.5)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	(0.2)	14.3	(13.1)	52.6
End of period	37.0	65.3	37.0	65.3
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	$ 0.0	0.2	$ 0.0	0.6
Collective account values of underlying variable annuities as a percentage of the guarantee value	107.00%		107.00%		107.00%
White Mountains Life Re | Level 2 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	$ 0.0		$ 0.0
Changes in derivative instruments
Beginning of period	32.5	22.3	33.8	4.7
Purchases		0.0		0.0
Realized and unrealized gains (losses)	3.2	(19.8)	(2.7)	(42.1)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	(7.8)	22.5	(3.2)	62.4
End of period	27.9	25.0	27.9	25.0
White Mountains Life Re | Level 1 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Changes in derivative instruments
Beginning of period	(5.1)	(0.8)	3.7	1.1
Purchases		0.0		0.0
Realized and unrealized gains (losses)	(5.1)	13.4	1.9	13.1
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	8.1	(8.2)	(7.7)	(9.8)
End of period	(2.1)	4.4	(2.1)	4.4
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		0.0
Changes in derivative instruments
Beginning balance	1.8	(31.8)	0.7	(52.8)
Realized and unrealized gains (losses)	(2.8)	20.1	(1.7)	41.1
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	0.0	0.0	0.0	0.0
Ending balance	(1.0)	(11.7)	(1.0)	(11.7)
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		0.0
Changes in derivative instruments
Beginning of period	8.3	53.3	18.9	63.4
Purchases		0.0		0.0
Realized and unrealized gains (losses)	3.4	(17.4)	(5.5)	(27.5)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	(0.5)	0.0	(2.2)	0.0
End of period	$ 11.2	$ 35.9	$ 11.2	$ 35.9