Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Private equity security
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 71.1
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	129.6	$ 125.3
Unfunded Commitments	131.3	58.1
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	58.5	54.9
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	58.5	54.9
Unfunded Commitments	0.0	0.0
Private equity security | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	39.2	43.3
Unfunded Commitments	10.5	10.4
Private equity security | Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.3	15.8
Unfunded Commitments	37.9	12.9
Private equity security | Direct Lending
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	7.8	7.1
Unfunded Commitments	22.6	23.1
Private equity security | Financial Services Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	5.8	4.2
Unfunded Commitments	10.3	11.7
Private equity security | Private equity security
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	71.1	70.4
Unfunded Commitments	131.3	58.1
Private equity security | Real estate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0	0.0
Unfunded Commitments	$ 50.0	$ 0.0