Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) $ in Millions	Mar. 31, 2018 USD ($) fund	Dec. 31, 2017 USD ($) fund
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments | fund	1
Hedge Funds Equity R E I T
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments		$ 20.8
September 2018 | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of hedge funds	1
Other long-term investments	$ 56.4
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	56.4	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	$ 56.4	$ 54.9
Number of investments | fund		1