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Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) $ in Millions, ¥ in Billions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2015 JPY (¥)
Changes in derivative instruments |  |  |  |  |  |
Collective account values of underlying variable annuities as a percentage of the guarantee value |  |  |  |  | 113.00% | 113.00%
White Mountains Life Re |  |  |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |
Notional amount |  |  |  |  | $ 400.0 | ¥ 50.7
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | $ 0.0 |  |  |
Changes in derivative instruments |  |  |  |  |  |
Beginning of period |  | $ 35.7 | 20.1 | $ 56.4 |  |
Purchases |  | 0.0 |  | 0.0 |  |
Realized and unrealized gains (losses) | $ 0.0 | 1.5 | (2.0) | (6.3) |  |
Transfers in |  | 0.0 | 0.0 | 0.0 |  |
Sales/settlements |  | (0.2) | (18.1) | (13.1) |  |
End of period |  | 37.0 |  | 37.0 |  |
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses | $ 0.0 | 0.0 | (0.3) | 0.0 |  |
Collective account values of underlying variable annuities as a percentage of the guarantee value |  |  |  |  | 109.00% | 109.00%
White Mountains Life Re | Level 2 Inputs |  |  |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | 0.0 |  |  |
Changes in derivative instruments |  |  |  |  |  |
Beginning of period |  | 32.5 | 16.5 | 33.8 |  |
Purchases |  | 0.0 |  | 0.0 |  |
Realized and unrealized gains (losses) |  | 3.2 | (0.7) | (2.7) |  |
Transfers in |  | 0.0 | 0.0 | 0.0 |  |
Sales/settlements |  | (7.8) | (15.8) | (3.2) |  |
End of period |  | 27.9 |  | 27.9 |  |
White Mountains Life Re | Level 1 Inputs |  |  |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | 0.0 |  |  |
Changes in derivative instruments |  |  |  |  |  |
Beginning of period |  | (5.1) | 0.9 | 3.7 |  |
Purchases |  | 0.0 |  | 0.0 |  |
Realized and unrealized gains (losses) |  | (5.1) | (4.2) | 1.9 |  |
Transfers in |  | 0.0 | 0.0 | 0.0 |  |
Sales/settlements |  | 8.1 | 3.3 | (7.7) |  |
End of period |  | (2.1) |  | (2.1) |  |
Variable Annuity | White Mountains Life Re | Level 3 Inputs |  |  |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | 0.0 |  |  |
Changes in derivative instruments |  |  |  |  |  |
Beginning balance |  | (1.8) | (0.3) | (0.7) |  |
Realized and unrealized gains (losses) |  | (2.8) | (0.3) | (1.7) |  |
Transfers in |  | 0.0 | 0.0 | 0.0 |  |
Sales/settlements |  | 0.0 | 0.0 | 0.0 |  |
Ending balance |  | 1.0 |  | 1.0 |  |
Purchases |  | 0.0 |  | 0.0 |  |
Derivative [Member] | White Mountains Life Re | Level 3 Inputs |  |  |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | 0.0 |  |  |
Changes in derivative instruments |  |  |  |  |  |
Beginning of period |  | 8.3 | 2.7 | 18.9 |  |
Purchases |  | 0.0 |  | 0.0 |  |
Realized and unrealized gains (losses) |  | 3.4 | 2.9 | (5.5) |  |
Transfers in |  | 0.0 | 0.0 | 0.0 |  |
Sales/settlements |  | (0.5) | $ (5.6) | (2.2) |  |
End of period |  | $ 11.2 |  | $ 11.2 |  |

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