Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) $ in Millions, ¥ in Billions	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 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	$ 800.0		$ 800.0		¥ 100.7	$ 1,100.0	¥ 134.2
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Changes in derivative instruments
Beginning balance	47.0	$ 89.6	56.4	$ 69.2
Purchases		0.0		0.0
Realized and unrealized gains (losses)	(2.3)	(26.2)	(7.8)	(32.7)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	(9.0)	11.4	(12.9)	38.3
Ending balance	35.7	74.8	35.7	74.8
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	$ 0.0	0.3	$ 0.0	0.3
Collective account values of underlying variable annuities as a percentage of the guarantee value	113.00%		113.00%		113.00%
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 balance	31.2	25.1	33.8	4.7
Purchases		0.0		0.0
Realized and unrealized gains (losses)	0.6	(15.5)	(5.9)	(22.5)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	0.7	12.7	4.6	40.1
Ending balance	32.5	22.3	32.5	22.3
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 balance	(1.2)	0.4	3.7	1.1
Purchases		0.0		0.0
Realized and unrealized gains (losses)	5.4	(0.1)	7.1	(0.3)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	(9.3)	(1.1)	(15.9)	(1.6)
Ending balance	(5.1)	(0.8)	(5.1)	(0.8)
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	1.1	(54.1)	0.7	(52.8)
Realized and unrealized gains (losses)	0.7	22.3	1.1	21.0
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	0.0	0.0	0.0	0.0
Ending balance	1.8	(31.8)	1.8	(31.8)
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 balance	17.0	64.1	18.9	63.4
Purchases		0.0		0.0
Realized and unrealized gains (losses)	(8.3)	(10.6)	(9.0)	(9.9)
Transfers in	0.0	0.0	0.0	0.0
Sales/settlements	(0.4)	(0.2)	(1.6)	(0.2)
Ending balance	$ 8.3	$ 53.3	$ 8.3	$ 53.3