Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Private equity funds
Investment securities
Trading security, fair value		$ 91.8
1 – 3 years | Private equity funds
Investment securities
Trading security, fair value		1.8
3 – 5 years | Private equity funds
Investment securities
Trading security, fair value		5.5
5 – 10 years | Private equity funds
Investment securities
Trading security, fair value		63.2
10 years | Private equity funds
Investment securities
Trading security, fair value		21.3
Hedge funds | Hedge funds
Investment securities
Trading security, fair value	$ 54.3	$ 54.3	$ 54.9
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period	45 years