Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Total	$ 38,665.0	$ 37,041.8
Gross Unrealized Gains	329.7	98.4
Gross Unrealized Losses	74.5	251.4
Fair Value	38,920.2	36,888.8
U.S. Government
Investment Holdings [Line Items]
Total	5,631.9	5,203.1
Gross Unrealized Gains	27.5	21.8
Gross Unrealized Losses	6.4	39.6
Fair Value	5,653.0	5,185.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	1,117.1	657.6
Gross Unrealized Gains	32.5	2.0
Gross Unrealized Losses	2.0	3.7
Fair Value	1,147.6	655.9
Government Sponsored Agency
Investment Holdings [Line Items]
Total	22,536.2	22,522.7
Gross Unrealized Gains	139.7	52.4
Gross Unrealized Losses	55.8	150.5
Fair Value	22,620.1	22,424.6
Non-U.S. Government
Investment Holdings [Line Items]
Total	3.3	143.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	1.1
Fair Value	3.3	142.2
Corporate Debt
Investment Holdings [Line Items]
Total	2,560.3	2,312.6
Gross Unrealized Gains	29.6	3.2
Gross Unrealized Losses	4.4	21.1
Fair Value	2,585.5	2,294.7
Covered Bonds
Investment Holdings [Line Items]
Total	759.4	832.7
Gross Unrealized Gains	5.0	1.4
Gross Unrealized Losses	0.2	4.8
Fair Value	764.2	829.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	2,062.2	2,087.8
Gross Unrealized Gains	43.5	11.9
Gross Unrealized Losses	0.9	3.5
Fair Value	2,104.8	2,096.2
Other Asset-Backed
Investment Holdings [Line Items]
Total	3,199.7	2,678.9
Gross Unrealized Gains	13.9	1.7
Gross Unrealized Losses	4.5	22.9
Fair Value	3,209.1	2,657.7
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	784.7	587.4
Gross Unrealized Gains	38.0	4.0
Gross Unrealized Losses	0.3	4.2
Fair Value	822.4	587.2
Other
Investment Holdings [Line Items]
Total	10.2	15.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 10.2	$ 15.7