Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions |
Sep. 30, 2018
USD ($)
fund
|
Dec. 31, 2017
USD ($)
|
|---|---|---|
| Hedge funds | ||
| Debt and Equity Securities, FV-NI [Line Items] | ||
| Number of investments | 1 | |
| Private equity security | ||
| Debt and Equity Securities, FV-NI [Line Items] | ||
| Number of investments | 13 | |
| Hedge and private equity funds included in other long-term investments | ||
| Debt and Equity Securities, FV-NI [Line Items] | ||
| Largest single fund investment | $ | $ 59.6 | $ 54.9 |