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Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Changes in derivative instruments |  |  |
Purchases | $ 6,381.7 | $ 2,377.1 |
Level 2 |  |  |
Changes in derivative instruments |  |  |
Purchases | 3,979.8 | 1,436.0 |
Level 1 |  |  |
Changes in derivative instruments |  |  |
Purchases | 2,243.7 | 814.5 |
White Mountains Life Re |  |  |
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 | 20.1 | 56.4 | $ 69.2
Purchases |  |  | 0.0
Realized and unrealized gains (losses) | 2.0 | 8.8 | 72.4
Transfers in (out) | 0.0 | 0.0 | 0.0
Sales/settlements | (18.1) | (27.5) | 59.6
Ending balance | 0.0 | 20.1 | 56.4
Collateral Cash and Investments | 0.0 | 5.8 | 33.2
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses | 0.0 | 0.0 | 0.6
White Mountains Life Re | Level 3 |  |  |
Changes in variable annuity (liabilities) |  |  |
Purchases |  |  | 0.0
White Mountains Life Re | 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 | 16.5 | 33.8 | 4.7
Purchases |  |  | 0.0
Realized and unrealized gains (losses) | 0.7 | 7.5 | 71.0
Transfers in (out) | 0.0 | 0.0 | 0.0
Sales/settlements | (15.8) | (9.8) | 100.1
Ending balance | 0.0 | 16.5 | 33.8
White Mountains Life Re | 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 | 0.9 | 3.7 | 1.1
Purchases |  |  | 0.0
Realized and unrealized gains (losses) | 4.2 | (8.4) | (37.2)
Transfers in (out) | 0.0 | 0.0 | 0.0
Sales/settlements | 3.3 | (11.2) | (34.6)
Ending balance | 0.0 | 0.9 | 3.7
Variable Annuity [Member] | White Mountains Life Re | 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 | 0.3 | 0.7 | 52.8
Net realized and unrealized (losses) gains | 0.3 | 0.4 | (53.5)
Transfers in (out) | 0.0 | 0.0 | 0.0
Sales/settlements | 0.0 | 0.0 | 0.0
Ending balance | 0.0 | 0.3 | 0.7
Derivative [Member] | White Mountains Life Re | 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 | 2.7 | 18.9 | 63.4
Purchases |  |  | 0.0
Realized and unrealized gains (losses) | (2.9) | 9.7 | 38.6
Transfers in (out) | 0.0 | 0.0 | 0.0
Sales/settlements | (5.6) | (6.5) | (5.9)
Ending balance | $ 0.0 | $ 2.7 | $ 18.9

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