Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 111.3
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	166.1	$ 146.1
Unfunded Commitments	72.6	76.7
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	54.8	54.3
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	54.8	54.3
Unfunded Commitments	0.0	0.0
Private equity security | Manufacturing/Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	53.2	42.9
Unfunded Commitments	4.1	10.5
Private equity security | Aerospace/Defense/Government
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	32.4	27.6
Unfunded Commitments	26.9	34.9
Private equity security | Direct lending
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	14.5	13.0
Unfunded Commitments	15.0	17.7
Private equity security | Financial services
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	11.2	8.3
Unfunded Commitments	26.6	13.6
Private equity security | Private equity security
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	111.3	91.8
Unfunded Commitments	$ 72.6	$ 76.7