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Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions | Sep. 30, 2017 USD ($) fund | Dec. 31, 2016 USD ($)
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Hedge funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Number of investments | 2 |
Private equity funds |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Number of investments | 10 |
Hedge and private equity funds included in other long-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Largest single fund investment | $ | $ 54.7 | $ 21.5

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