Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 58.4
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	84.6
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	143.0	$ 127.8
Unfunded Commitments	129.6	82.2
Private equity funds | Private Equity Energy Infrastructure and Services
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	14.2	20.7
Unfunded Commitments	3.2	3.4
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	10.4	14.8
Unfunded Commitments	2.0	2.1
Private equity funds | Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.7	4.4
Unfunded Commitments	2.1	2.1
Private equity funds | Private Equity Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.8	3.8
Unfunded Commitments	0.4	0.4
Private equity funds | Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.8	2.0
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	23.1	19.8
Unfunded Commitments	21.8	30.3
Private equity funds | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	27.7	24.9
Unfunded Commitments	21.8	2.5
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	84.6	90.8
Unfunded Commitments	129.6	82.2
Private equity funds | Real Estate Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.3	0.4
Unfunded Commitments	0.1	0.1
Private equity funds | Venture Capital Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.6	0.0
Unfunded Commitments	36.9	0.0
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	34.6	12.8
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	20.4	20.6
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	3.4	3.6
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	58.4	37.0
Unfunded Commitments	$ 0.0	$ 0.0