Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Total	$ 39,025.3	$ 38,722.2
Gross Unrealized Gains	657.0	262.7
Gross Unrealized Losses	240.6	108.6
Fair Value	39,441.7	38,876.3
U.S. Government
Investment Holdings [Line Items]
Total	4,481.7	4,527.5
Gross Unrealized Gains	102.4	26.7
Gross Unrealized Losses	0.0	5.1
Fair Value	4,584.1	4,549.1
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	1,725.2	1,604.0
Gross Unrealized Gains	54.3	24.6
Gross Unrealized Losses	3.2	13.3
Fair Value	1,776.3	1,615.3
Government Sponsored Agency
Investment Holdings [Line Items]
Total	23,611.0	23,247.5
Gross Unrealized Gains	365.3	101.8
Gross Unrealized Losses	106.3	78.1
Fair Value	23,870.0	23,271.2
Non-U.S. Government
Investment Holdings [Line Items]
Total	3.3	3.3
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	3.4	3.3
Corporate Debt
Investment Holdings [Line Items]
Total	2,408.6	2,378.9
Gross Unrealized Gains	38.7	27.8
Gross Unrealized Losses	11.3	4.0
Fair Value	2,436.0	2,402.7
Covered Bonds
Investment Holdings [Line Items]
Total	550.5	766.3
Gross Unrealized Gains	4.2	4.4
Gross Unrealized Losses	1.3	0.8
Fair Value	553.4	769.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	2,047.5	2,091.3
Gross Unrealized Gains	58.6	37.4
Gross Unrealized Losses	0.6	1.1
Fair Value	2,105.5	2,127.6
Other Asset-Backed
Investment Holdings [Line Items]
Total	3,284.9	3,324.5
Gross Unrealized Gains	2.3	11.3
Gross Unrealized Losses	105.1	5.3
Fair Value	3,182.1	3,330.5
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	904.3	769.9
Gross Unrealized Gains	31.1	28.7
Gross Unrealized Losses	12.8	0.9
Fair Value	922.6	797.7
Other
Investment Holdings [Line Items]
Total	8.3	9.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 8.3	$ 9.0