Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 72.1
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	76.4
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	148.5	$ 82.6
Unfunded Commitments	107.4	123.6
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	53.8	21.5
Unfunded Commitments	0.0	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	18.3	19.9
Unfunded Commitments	0.0	0.0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	72.1	41.4
Unfunded Commitments	0.0	0.0
Private equity funds | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	45.2	19.4
Unfunded Commitments	13.1	22.9
Private equity funds | Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	24.6	19.4
Unfunded Commitments	23.6	25.9
Private equity funds | Direct Lending
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	5.1	1.4
Unfunded Commitments	25.0	28.6
Private equity funds | Financial Services Sector
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 Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.0	0.0
Unfunded Commitments	41.2	41.2
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	76.4	41.2
Unfunded Commitments	$ 107.4	$ 123.6