Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 36.4
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	94.9
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	131.3	$ 127.8
Unfunded Commitments	73.5	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	18.0	20.7
Unfunded Commitments	3.3	3.4
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	13.0	14.8
Unfunded Commitments	2.1	2.1
Private equity funds | Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.9	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	2.0	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	27.9	19.8
Unfunded Commitments	22.1	30.3
Private equity funds | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	25.8	24.9
Unfunded Commitments	2.1	2.5
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	94.9	90.8
Unfunded Commitments	73.5	82.2
Private equity funds | Real Estate Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.5	0.4
Unfunded Commitments	0.1	0.1
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	12.6	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	20.2	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	3.6	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	36.4	37.0
Unfunded Commitments	$ 0.0	$ 0.0