Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 55.6
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	93.0
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	148.6	$ 178.5
Unfunded Commitments	86.1	64.3
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	14.9	14.5
Unfunded Commitments	2.1	2.2
Private equity funds | Private Equity Energy Infrastructure and Services
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	26.3	33.1
Unfunded Commitments	4.0	4.8
Private equity funds | Real Estate Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.8	1.7
Unfunded Commitments	0.1	0.1
Private equity funds | Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.5	5.7
Unfunded Commitments	2.1	2.1
Private equity funds | Private Equity Funds Manufacturing and Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	23.5	23.2
Unfunded Commitments	2.5	7.3
Private equity funds | Private Equity Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.9	3.1
Unfunded Commitments	0.4	1.4
Private equity funds | Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.0	2.1
Unfunded Commitments	41.3	41.3
Private equity funds | Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	17.1	20.7
Unfunded Commitments	33.6	5.1
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	93.0	104.1
Unfunded Commitments	86.1	64.3
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	15.4	29.9
Hedge funds | Hedge Funds, Equity Long (Short) [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge Funds Equity R E I T [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.0	20.4
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	12.1	15.7
Hedge funds | Hedge Funds Bank Loan [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge Funds Credit and Distressed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	8.1	8.4
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	55.6	74.4
Unfunded Commitments	$ 0.0	$ 0.0