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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

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