Derivatives Realized and Unrealized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Variable Annuity Reinsurance [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 6.8	$ 0.0	$ 14.1
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	0.0		0.0			$ 20.1
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(0.3)	(2.3)	(2.0)	(7.8)
Derivative, fair value, net	0.0	35.7	0.0	35.7	$ 13.1	20.1	$ 47.0	$ 56.4
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	12.8	9.0	18.1	12.9
Fixed income/interest rate | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0.0	(5.9)	1.8	3.4
Derivative, fair value, net	0.0		0.0			0.5
Foreign exchange | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(0.6)	9.2	(4.8)	(2.9)
Derivative, fair value, net	0.0		0.0			14.8
Equity | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0.3	(5.6)	1.0	(8.3)
Derivative, fair value, net	0.0		0.0			4.8
Level 2 Inputs | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(1.8)	0.6	(0.7)	(5.9)
Derivative, fair value, net	0.0	32.5	0.0	32.5	10.3	16.5	31.2	33.8
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	8.5	(0.7)	15.8	(4.6)
Level 1 Inputs | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(0.2)	5.4	(4.2)	7.1
Derivative, fair value, net	0.0	(5.1)	0.0	(5.1)	0.2	0.9	(1.2)	3.7
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		9.3	(3.3)	15.9
Fair Value, Inputs, Level 3 [Member] | Variable Annuity | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	0.0	(1.8)	0.0	(1.8)	(0.1)	(0.3)	(1.1)	(0.7)
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	0.1	0.7	(0.3)	1.1
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] | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	1.7	(8.3)	2.9	(9.0)
Derivative, fair value, net	0.0	8.3	0.0	8.3	$ 2.6	$ 2.7	$ 17.0	$ 18.9
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	$ 4.3	$ 0.4	$ 5.6	$ 1.6