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Derivatives Realized and Unrealized Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2015 | Jun. 30, 2015 | Dec. 31, 2014
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax | $ 0.0 | $ 3.9 | $ 0.0 | $ 18.0 |  |  |
White Mountains Life Re |  |  |  |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |  |
Derivative, Fair Value, Amount Offset Against Collateral, Net |  |  |  |  | $ 20.1 |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |  |  |
Realized and unrealized gains (losses) | 0.0 | 1.5 | (2.0) | (6.3) |  |  |
Derivative, fair value, net |  | 37.0 |  | 37.0 | 20.1 | $ 35.7 | $ 56.4
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | 0.0 |  |  |  |
Derivative Transfers In (Out), Net |  | 0.0 | 0.0 | 0.0 |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements |  | 0.2 | 18.1 | 13.1 |  |  |
Fixed income/interest rate | White Mountains Life Re |  |  |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |  |  |
Realized and unrealized gains (losses) | 0.0 | 2.6 | 1.8 | 6.0 |  |  |
Derivative, fair value, net |  |  |  |  | 0.5 |  |
Foreign exchange | White Mountains Life Re |  |  |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |  |  |
Realized and unrealized gains (losses) | 0.0 | (3.1) | (4.8) | (6.0) |  |  |
Derivative, fair value, net |  |  |  |  | 14.8 |  |
Equity | White Mountains Life Re |  |  |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |  |  |
Realized and unrealized gains (losses) | $ 0.0 | 2.0 | 1.0 | (6.3) |  |  |
Derivative, fair value, net |  |  |  |  | 4.8 |  |
Level 2 Inputs | White Mountains Life Re |  |  |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |  |  |
Realized and unrealized gains (losses) |  | 3.2 | (0.7) | (2.7) |  |  |
Derivative, fair value, net |  | 27.9 |  | 27.9 | 16.5 | 32.5 | 33.8
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | 0.0 |  |  |  |
Derivative Transfers In (Out), Net |  | 0.0 | 0.0 | 0.0 |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements |  | 7.8 | 15.8 | 3.2 |  |  |
Level 1 Inputs | White Mountains Life Re |  |  |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |  |  |
Realized and unrealized gains (losses) |  | (5.1) | (4.2) | 1.9 |  |  |
Derivative, fair value, net |  | (2.1) |  | (2.1) | 0.9 | (5.1) | 3.7
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | 0.0 |  |  |  |
Derivative Transfers In (Out), Net |  | 0.0 | 0.0 | 0.0 |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements |  | (8.1) | (3.3) | 7.7 |  |  |
Fair Value, Inputs, Level 3 [Member] | Variable Annuity | White Mountains Life Re |  |  |  |  |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |  |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value |  | 1.0 |  | 1.0 | (0.3) | (1.8) | (0.7)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | 0.0 |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings |  | (2.8) | (0.3) | (1.7) |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers |  | 0.0 | 0.0 | 0.0 |  |  |
Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis, Liability Sales or Settlements |  | 0.0 | 0.0 | 0.0 |  |  |
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | White Mountains Life Re |  |  |  |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |  |  |  |
Realized and unrealized gains (losses) |  | 3.4 | 2.9 | (5.5) |  |  |
Derivative, fair value, net |  | 11.2 |  | 11.2 | $ 2.7 | $ 8.3 | $ 18.9
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  |  | 0.0 |  |  |  |
Derivative Transfers In (Out), Net |  | 0.0 | 0.0 | 0.0 |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements |  | $ 0.5 | $ 5.6 | $ 2.2 |  |  |

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