Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) - White Mountains Life Re - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	56.4	69.2	$ 98.3
Purchases			59.4
Realized and unrealized gains (losses)	8.8	72.4	402.0
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	27.5	(59.6)	313.5
Ending balance	20.1	56.4	69.2
Collateral Cash and Investments	5.8	33.2	81.3
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses	0.0	0.6	10.2
Level 3
Changes in variable annuity (liabilities)
Purchases			0.0
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	33.8	4.7	(20.5)
Purchases			0.0
Realized and unrealized gains (losses)	7.5	71.0	196.1
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	9.8	(100.1)	221.3
Ending balance	16.5	33.8	4.7
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	3.7	1.1	(21.7)
Purchases			0.0
Realized and unrealized gains (losses)	(8.4)	(37.2)	69.4
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	11.2	34.6	92.2
Ending balance	0.9	3.7	1.1
Variable Annuity [Member] | 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	(0.7)	(52.8)	(441.5)
Realized and unrealized (losses) gains	0.4	(53.5)	(388.7)
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	0.0	0.0	0.0
Ending balance	(0.3)	(0.7)	(52.8)
Derivative [Member] | 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	18.9	63.4	140.5
Purchases			59.4
Realized and unrealized gains (losses)	9.7	38.6	136.5
Transfers in (out)	0.0	0.0	0.0
Sales/settlements	6.5	5.9	0.0
Ending balance	$ 2.7	$ 18.9	$ 63.4