Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) $ in Millions	Jun. 30, 2018 USD ($) fund	Dec. 31, 2017 USD ($)
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments | fund	1
September 2018 | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of hedge funds	1
Other long-term investments	$ 58.5
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	58.5	$ 54.9
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 58.5	$ 54.9