Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 81.1
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	154.9	$ 82.6
Unfunded Commitments	105.2	123.6
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	54.7	21.5
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge Funds Equity R E I T
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	19.1	19.9
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	73.8	41.4
Unfunded Commitments	0.0	0.0
Private equity funds | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	42.7	19.4
Unfunded Commitments	12.1	22.9
Private equity funds | Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	28.7	19.4
Unfunded Commitments	15.6	25.9
Private equity funds | Direct Lending
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	6.7	1.4
Unfunded Commitments	23.3	28.6
Private equity funds | Financial Services Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.0	1.0
Unfunded Commitments	13.0	5.0
Private equity funds | Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0	0.0
Unfunded Commitments	41.2	41.2
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	81.1	41.2
Unfunded Commitments	$ 105.2	$ 123.6