Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Amortized Cost	$ 31,007.6	$ 29,514.1
Gross unrealized, gains	123.3	110.9
Gross unrealized, losses	39.2	66.5
Fair Value	31,091.7	29,558.5
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	5,094.0	4,493.5
Gross unrealized, gains	31.1	15.1
Gross unrealized, losses	0.0	1.7
Fair Value	5,125.1	4,506.9
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	4.5	4.5
Gross unrealized, gains	0.0	0.1
Gross unrealized, losses	0.0	0.0
Fair Value	4.5	4.6
Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	15,637.4	16,326.4
Gross unrealized, gains	81.2	82.3
Gross unrealized, losses	12.7	19.5
Fair Value	15,705.9	16,389.2
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	310.8	309.5
Gross unrealized, gains	0.2	0.9
Gross unrealized, losses	0.0	0.0
Fair Value	311.0	310.4
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,810.7	3,617.5
Gross unrealized, gains	4.0	1.8
Gross unrealized, losses	20.1	41.6
Fair Value	3,794.6	3,577.7
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	2,073.2	1,899.9
Gross unrealized, gains	3.3	7.9
Gross unrealized, losses	3.0	0.3
Fair Value	2,073.5	1,907.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	834.8
Gross unrealized, gains	1.5
Gross unrealized, losses	0.2
Fair Value	836.1
Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost		360.0
Gross unrealized, gains		1.5
Gross unrealized, losses		0.9
Fair Value		360.6
Residential Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost		6.9
Gross unrealized, gains		0.0
Gross unrealized, losses		0.5
Fair Value		6.4
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,817.2	2,321.8
Gross unrealized, gains	0.6	0.5
Gross unrealized, losses	2.1	1.0
Fair Value	2,815.7	2,321.3
Auction Rate
Investment Holdings [Line Items]
Amortized Cost	16.7	18.4
Gross unrealized, gains	0.5	0.5
Gross unrealized, losses	1.0	0.8
Fair Value	16.2	18.1
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	298.8
Gross unrealized, gains	0.7
Gross unrealized, losses	0.1
Fair Value	299.4
Other
Investment Holdings [Line Items]
Amortized Cost	109.5	155.7
Gross unrealized, gains	0.2	0.3
Gross unrealized, losses	0.0	0.2
Fair Value	$ 109.7	$ 155.8