Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 76.1
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	71.1
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	147.2	$ 131.0
Unfunded Commitments	131.2	138.1
Private equity funds | Private Equity Energy Infrastructure and Services
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.8	14.1
Unfunded Commitments	3.0	3.2
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	11.0	11.4
Unfunded Commitments	2.0	2.0
Private equity funds | Direct Lending [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	5.5	1.8
Unfunded Commitments	32.0	35.7
Private equity funds | Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.5	3.0
Unfunded Commitments	2.1	2.1
Private equity funds | Financial Services Sector [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	1.5	1.0
Unfunded Commitments	4.5	5.0
Private equity funds | Private Equity Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	3.4	3.5
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	0.9	0.8
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	24.4	19.4
Unfunded Commitments	23.6	25.9
Private equity funds | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.8	15.9
Unfunded Commitments	22.2	22.4
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	71.1	71.2
Unfunded Commitments	131.2	138.1
Private equity funds | Real Estate Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.3	0.3
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	54.2	36.5
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	19.7	19.9
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	2.2	3.4
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	76.1	59.8
Unfunded Commitments	$ 0.0	$ 0.0