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Derivatives Forward Contracts (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2017 | Jun. 30, 2017 | Dec. 31, 2016
Forward Contracts |  |  |
Derivative [Line Items] |  |  |
Notional amount | $ 302.0 | $ 302.0 |
Realized and unrealized gains (losses) | (1.7) | 1.7 |
Unrealized gain (loss) on derivatives | 8.5 | 11.3 |
Derivative, fair value, net | (12.5) | (12.5) |
Other Investments |  |  |
Derivative [Line Items] |  |  |
Derivative liability | 12.5 | 12.5 | $ 1.2
United States of America, Dollars |  |  |
Derivative [Line Items] |  |  |
Notional amount | 302.0 | 302.0 |
Derivative, fair value, net | 57.0 | 57.0 |
United Kingdom, Pounds |  |  |
Derivative [Line Items] |  |  |
Notional amount | 150.0 | 150.0 |
Euro Member Countries, Euro |  |  |
Derivative [Line Items] |  |  |
Notional amount | 104.0 | 104.0 |
Derivative, fair value, net | 50.0 | 50.0 |
Barclays | Standard & Poor's, A- Rating | Forward Contracts |  |  |
Derivative [Line Items] |  |  |
Notional amount | 187.3 | 187.3 |
Derivative, fair value, net | (8.2) | (8.2) |
JP Morgan | United States of America, Dollars |  |  |
Derivative [Line Items] |  |  |
Notional amount | 52.7 | 52.7 |
JP Morgan | Standard & Poor's, A plus | Forward Contracts |  |  |
Derivative [Line Items] |  |  |
Notional amount | 114.7 | 114.7 |
Derivative, fair value, net | $ (4.3) | $ (4.3) |

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