Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 105.9
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	105.9	$ 161.1
Unfunded Commitments	44.4	50.2
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	0.0	54.6
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	0.0	54.6
Unfunded Commitments	0.0	0.0
Private equity security | Manufacturing/Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	43.8	57.7
Unfunded Commitments	4.0	4.1
Private equity security | Aerospace/Defense/Government
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	44.8	33.8
Unfunded Commitments	15.7	23.3
Private equity security | Financial services
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	17.3	15.0
Unfunded Commitments	24.7	22.8
Private equity security | Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	105.9	106.5
Unfunded Commitments	$ 44.4	$ 50.2