Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - Advance Notice Period [Domain] - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 77.7
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	133.1
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	210.8	$ 242.9
Unfunded Commitments	94.8	81.7
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	24.5	24.2
Unfunded Commitments	4.4	5.3
Private equity funds | Private Equity Energy Infrastructure and Services
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	45.7	59.6
Unfunded Commitments	9.9	11.0
Private equity funds | Real Estate Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	1.8	3.6
Unfunded Commitments	0.1	3.3
Private equity funds | Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	7.4	8.5
Unfunded Commitments	3.2	3.1
Private equity funds | Private Equity Funds Multi Sector Europe
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.4	1.5
Unfunded Commitments	0.0	2.3
Private equity funds | Private Equity Funds Manufacturing and Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	25.9	23.2
Unfunded Commitments	2.9	7.3
Private equity funds | Private Equity Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	6.3	6.1
Unfunded Commitments	0.8	2.8
Private equity funds | Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.1	2.1
Unfunded Commitments	41.3	41.2
Private equity funds | Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.7	20.7
Unfunded Commitments	32.2	5.1
Private equity funds | Venture Capital Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.3	1.4
Unfunded Commitments	0.0	0.3
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	133.1	150.9
Unfunded Commitments	94.8	81.7
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	15.9	29.9
Hedge funds | Hedge Funds Equity R E I T [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.5	20.3
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge Funds, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	22.1	20.4
Hedge funds | Hedge Funds Credit and Distressed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	19.2	21.4
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	77.7	92.0
Unfunded Commitments	$ 0.0	$ 0.0