Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) $ in Millions	Dec. 31, 2018 USD ($) fund	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Private equity funds
Investment securities
Number of investments held | fund	12
Trading security, fair value		$ 91.8
Hedge funds
Investment securities
Number of investments held | fund	1
Total private equity funds and hedge funds included in other long-term investments
Investment securities
Other long-term investments largest single investment	$ 54.3		$ 54.9
Trading security, fair value	146.1		125.3
Fair value unfunded commitments	76.7		58.1
Private equity funds | Private equity funds
Investment securities
Trading security, fair value		91.8	70.4
Fair value unfunded commitments	76.7		58.1
Private equity funds | Manufacturing/Industrial
Investment securities
Trading security, fair value		42.9	43.3
Fair value unfunded commitments	10.5		10.4
Private equity funds | Aerospace/Defense/Government
Investment securities
Trading security, fair value		27.6	15.8
Fair value unfunded commitments	34.9		12.9
Private equity funds | Direct lending
Investment securities
Trading security, fair value		13.0	7.1
Fair value unfunded commitments	17.7		23.1
Private equity funds | Financial services
Investment securities
Trading security, fair value		8.3	4.2
Fair value unfunded commitments	13.6		11.7
Hedge funds | Hedge funds
Investment securities
Trading security, fair value	54.3	54.3	54.9
Fair value unfunded commitments	0.0		0.0
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Investment securities
Trading security, fair value		$ 54.3	54.9
Fair value unfunded commitments	$ 0.0		$ 0.0