Derivatives Realized and Unrealized Gains (Losses) (Details) - White Mountains Life Re - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	$ 1.5	$ (23.8)	$ (6.3)	$ (56.5)
Derivative, fair value, net	37.0	65.3	37.0	65.3	$ 35.7	$ 56.4	$ 74.8	$ 69.2
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Derivative Transfers In (Out), Net	0.0	0.0	0.0	0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	0.2	(14.3)	13.1	(52.6)
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	2.6	(8.4)	6.0	(20.4)
Derivative, fair value, net	0.0		0.0			(1.7)
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(3.1)	(0.4)	(6.0)	(13.3)
Derivative, fair value, net	27.9		27.9			44.1
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	2.0	(15.0)	(6.3)	(22.8)
Derivative, fair value, net	9.1		9.1			14.0
Level 2 Inputs
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	3.2	(19.8)	(2.7)	(42.1)
Derivative, fair value, net	27.9	25.0	27.9	25.0	32.5	33.8	22.3	4.7
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Derivative Transfers In (Out), Net	0.0	0.0	0.0	0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	7.8	(22.5)	3.2	(62.4)
Level 1 Inputs
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(5.1)	13.4	1.9	13.1
Derivative, fair value, net	(2.1)	4.4	(2.1)	4.4	(5.1)	3.7	(0.8)	1.1
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Derivative Transfers In (Out), Net	0.0	0.0	0.0	0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	(8.1)	8.2	7.7	9.8
Fair Value, Inputs, Level 3 [Member] | Variable Annuity
Variable Annuity Reinsurance [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	(1.0)	(11.7)	(1.0)	(11.7)	1.8	0.7	(31.8)	(52.8)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	(2.8)	20.1	(1.7)	41.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers	0.0	0.0	0.0	0.0
Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis, Liability Sales or Settlements	0.0	0.0	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Derivative [Member]
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	3.4	(17.4)	(5.5)	(27.5)
Derivative, fair value, net	11.2	35.9	11.2	35.9	$ 8.3	$ 18.9	$ 53.3	$ 63.4
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0.0		0.0
Derivative Transfers In (Out), Net	0.0	0.0	0.0	0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	$ 0.5	$ 0.0	$ 2.2	$ 0.0