Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 97.6
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150.7	$ 146.1
Unfunded Commitments	69.9	76.7
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	53.1	54.3
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	53.1	54.3
Unfunded Commitments	0.0	0.0
Private equity security | Manufacturing/Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	42.1	42.9
Unfunded Commitments	10.5	10.5
Private equity security | Aerospace/Defense/Government
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	32.8	27.6
Unfunded Commitments	29.8	34.9
Private equity security | Direct lending
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	13.6	13.0
Unfunded Commitments	16.8	17.7
Private equity security | Financial services
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	9.1	8.3
Unfunded Commitments	12.8	13.6
Private equity security | Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	97.6	91.8
Unfunded Commitments	$ 69.9	$ 76.7