Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 92.1
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	150.4
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	242.5	242.9
Unfunded Commitments	70.7	81.7
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	24.3	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	51.3	59.6
Unfunded Commitments	10.0	11.0
Private equity funds | Real Estate Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.0	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	8.0	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	1.2	1.5
Unfunded Commitments	2.1	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	30.5	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	8.7	6.1
Unfunded Commitments	1.3	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	20.9	20.7
Unfunded Commitments	5.1	5.1
Private equity funds | Venture Capital Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	1.4	1.4
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	150.4	150.9
Unfunded Commitments	70.7	81.7
Hedge funds | Hedge Funds, Equity Long (Short) [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	44.3	43.9
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	20.0	21.4
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	21.0	20.3
Unfunded Commitments	0	0
Hedge funds | Hedge Funds Equity Activist
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	6.6	6.2
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	92.1	92.0
Unfunded Commitments	$ 0	$ 0