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Derivatives Forward Contracts (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2017 | Jun. 30, 2017 | Sep. 30, 2017 | Dec. 31, 2016
Forward Contracts |  |  |  |
Derivative [Line Items] |  |  |  |
Notional amount | $ 261.8 |  | $ 261.8 |
Realized and unrealized gains (losses) | 7.2 | $ 1.7 | 8.9 |
Unrealized gain (loss) on derivatives | 2.8 |  | 14.2 |
Derivative, fair value, net | (15.4) |  | (15.4) |
Other Investments |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative liability | 15.4 |  | 15.4 | $ 1.2
United States of America, Dollars |  |  |  |
Derivative [Line Items] |  |  |  |
Notional amount | 261.8 |  | 261.8 |
United Kingdom, Pounds |  |  |  |
Derivative [Line Items] |  |  |  |
Notional amount | 160.7 |  | 160.7 |
Euro Member Countries, Euro |  |  |  |
Derivative [Line Items] |  |  |  |
Notional amount | 18.9 |  | 18.9 |
Japan, Yen |  |  |  |
Derivative [Line Items] |  |  |  |
Notional amount | 2,646.4 |  | 2,646.4 |
Barclays | Standard & Poor's, A- Rating | Forward Contracts |  |  |  |
Derivative [Line Items] |  |  |  |
Notional amount | 201.3 |  | 201.3 |
Derivative, fair value, net | (14.1) |  | (14.1) |
JP Morgan | Standard & Poor's, A plus | Forward Contracts |  |  |  |
Derivative [Line Items] |  |  |  |
Notional amount | 60.5 |  | 60.5 |
Derivative, fair value, net | $ (1.3) |  | $ (1.3) |

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