Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in derivative instruments
Purchases	$ 6,381.7	$ 2,377.1
Level 2
Changes in derivative instruments
Purchases	3,979.8	1,436.0
Level 1
Changes in derivative instruments
Purchases	2,243.7	814.5
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in derivative instruments
Beginning of period	20.1	56.4	$ 69.2
Purchases			0.0
Realized and unrealized gains (losses)	2.0	8.8	72.4
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	(18.1)	(27.5)	59.6
Ending balance	0.0	20.1	56.4
Collateral Cash and Investments	0.0	5.8	33.2
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses	0.0	0.0	0.6
White Mountains Life Re | Level 3
Changes in variable annuity (liabilities)
Purchases			0.0
White Mountains Life Re | Level 2
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in derivative instruments
Beginning of period	16.5	33.8	4.7
Purchases			0.0
Realized and unrealized gains (losses)	0.7	7.5	71.0
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	(15.8)	(9.8)	100.1
Ending balance	0.0	16.5	33.8
White Mountains Life Re | Level 1
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in derivative instruments
Beginning of period	0.9	3.7	1.1
Purchases			0.0
Realized and unrealized gains (losses)	4.2	(8.4)	(37.2)
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	3.3	(11.2)	(34.6)
Ending balance	0.0	0.9	3.7
Variable Annuity [Member] | White Mountains Life Re | Level 3
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in variable annuity (liabilities)
Beginning balance	0.3	0.7	52.8
Net realized and unrealized (losses) gains	0.3	0.4	(53.5)
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	0.0	0.0	0.0
Ending balance	0.0	0.3	0.7
Derivative [Member] | White Mountains Life Re | Level 3
Variable Annuity Reinsurance [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0
Changes in derivative instruments
Beginning of period	2.7	18.9	63.4
Purchases			0.0
Realized and unrealized gains (losses)	(2.9)	9.7	38.6
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	(5.6)	(6.5)	(5.9)
Ending balance	$ 0.0	$ 2.7	$ 18.9