Derivatives Realized and Unrealized Gains (Losses) (Details) - White Mountains Life Re - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	$ 0.0	$ (2.0)
Derivative, fair value, net	0.0	0.0	$ 0.0	$ 20.1
Purchases		0.0
Derivative Transfers In (Out), Net		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		18.1
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0.0	1.8
Derivative, fair value, net			0.0
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0.0	(4.8)
Derivative, fair value, net			0.0
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0.0	1.0
Derivative, fair value, net			$ 0.0
Level 3 Inputs | Variable Annuity
Variable Annuity Reinsurance [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	0.0	0.0		(0.3)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Purchases		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		(0.3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers		0.0
Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis, Liability Sales or Settlements		0.0
Level 3 Inputs | Derivative
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)		2.9
Derivative, fair value, net	0.0	0.0		2.7
Purchases		0.0
Derivative Transfers In (Out), Net		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		5.6
Level 2 Inputs
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)		(0.7)
Derivative, fair value, net	0.0	0.0		16.5
Purchases		0.0
Derivative Transfers In (Out), Net		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		15.8
Level 1 Inputs
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)		(4.2)
Derivative, fair value, net	$ 0.0	0.0		$ 0.9
Purchases		0.0
Derivative Transfers In (Out), Net		0.0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements		$ (3.3)