Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 114.6
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	127.3
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	241.9	239.0
Unfunded Commitments	98.4	110.6
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	24.5	23.8
Unfunded Commitments	5.9	6.5
Private equity funds | Private Equity Energy Infrastructure and Services
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	46.8	45.9
Unfunded Commitments	11.0	13.1
Private equity funds | Real Estate Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.5	8.2
Unfunded Commitments	3.3	3.3
Private equity funds | Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	9.4	9.5
Unfunded Commitments	3.1	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	2.8	3.9
Unfunded Commitments	2.5	2.8
Private equity funds | Private Equity Distressed Residential Real Estate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0	0.4
Unfunded Commitments	0	0
Private equity funds | Private Equity Funds Manufacturing and Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.8	11.2
Unfunded Commitments	14.8	15.5
Private equity funds | Private Equity Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	5.8	5.6
Unfunded Commitments	2.8	2.8
Private equity funds | Private Equity Funds Multi Sector Asia
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0	0
Unfunded Commitments	0	2.7
Private equity funds | Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.2	2.3
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	12.1	5.8
Unfunded Commitments	13.4	19.2
Private equity funds | Venture Capital Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	1.4	1.6
Unfunded Commitments	0.3	0.3
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	127.3	118.2
Unfunded Commitments	98.4	110.6
Hedge funds | Hedge Funds, Equity Long (Short) [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	54.8	62.6
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Credit and Distressed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	22.9	22.8
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Diversified Strategies
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0	0.1
Unfunded Commitments	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	19.5	18.3
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Equity Activist
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	17.2	16.8
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Bank Loan
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.2	0.2
Unfunded Commitments	0	0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	114.6	120.8
Unfunded Commitments	$ 0	$ 0