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Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Changes in derivative instruments |  |  |
Purchases | $ 3,332.5 | $ 4,728.3 |
Level 2 |  |  |
Changes in derivative instruments |  |  |
Purchases | 2,007.9 | 2,868.8 |
Level 1 |  |  |
Changes in derivative instruments |  |  |
Purchases | 1,209.3 | 1,746.9 |
White Mountains Life Re |  |  |
Changes in derivative instruments |  |  |
Beginning of period | 0.0 | 20.1 | $ 56.4
Purchases |  | 0.0 | 0.0
Realized and unrealized gains (losses) |  | 2.0 | 8.8
Transfers in (out) |  | 0.0 | 0.0
Sales/settlements |  | (18.1) | (27.5)
Ending balance |  | 0.0 | 20.1
Collateral Cash and Investments |  |  | 5.8
White Mountains Life Re | Level 3 |  |  |
Changes in variable annuity (liabilities) |  |  |
Purchases |  |  | 0.0
White Mountains Life Re | Level 2 |  |  |
Changes in derivative instruments |  |  |
Beginning of period | 0.0 | 16.5 | 33.8
Purchases |  | 0.0 | 0.0
Realized and unrealized gains (losses) |  | 0.7 | 7.5
Transfers in (out) |  | 0.0 | 0.0
Sales/settlements |  | (15.8) | (9.8)
Ending balance |  | 0.0 | 16.5
White Mountains Life Re | Level 1 |  |  |
Changes in derivative instruments |  |  |
Beginning of period | 0.0 | 0.9 | 3.7
Purchases |  | 0.0 | 0.0
Realized and unrealized gains (losses) |  | 4.2 | (8.4)
Transfers in (out) |  | 0.0 | 0.0
Sales/settlements |  | 3.3 | (11.2)
Ending balance |  | 0.0 | 0.9
Variable Annuity | White Mountains Life Re | Level 3 |  |  |
Changes in variable annuity (liabilities) |  |  |
Beginning balance | 0.0 | 0.3 | (0.7)
Purchases |  | 0.0 |
Net realized and unrealized (losses) gains |  | 0.3 | 0.4
Transfers in (out) |  | 0.0 | 0.0
Sales/settlements |  | 0.0 | 0.0
Ending balance |  | 0.0 | 0.3
Derivative | White Mountains Life Re | Level 3 |  |  |
Changes in derivative instruments |  |  |
Beginning of period | $ 0.0 | 2.7 | 18.9
Purchases |  | 0.0 | 0.0
Realized and unrealized gains (losses) |  | (2.9) | 9.7
Transfers in (out) |  | 0.0 | 0.0
Sales/settlements |  | (5.6) | (6.5)
Ending balance |  | $ 0.0 | $ 2.7

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