Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Total	$ 37,686.1	$ 37,041.8
Gross Unrealized Gains	269.6	98.4
Gross Unrealized Losses	88.2	251.4
Fair Value	37,867.5	36,888.8
U.S. Government
Investment Holdings [Line Items]
Total	5,065.5	5,203.1
Gross Unrealized Gains	23.9	21.8
Gross Unrealized Losses	10.0	39.6
Fair Value	5,079.4	5,185.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	730.5	657.6
Gross Unrealized Gains	22.8	2.0
Gross Unrealized Losses	0.1	3.7
Fair Value	753.2	655.9
Government Sponsored Agency
Investment Holdings [Line Items]
Total	22,456.1	22,522.7
Gross Unrealized Gains	113.0	52.4
Gross Unrealized Losses	63.7	150.5
Fair Value	22,505.4	22,424.6
Non-U.S. Government
Investment Holdings [Line Items]
Total	143.3	143.3
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.2	1.1
Fair Value	143.2	142.2
Corporate Debt
Investment Holdings [Line Items]
Total	2,404.1	2,312.6
Gross Unrealized Gains	23.8	3.2
Gross Unrealized Losses	6.1	21.1
Fair Value	2,421.8	2,294.7
Covered Bonds
Investment Holdings [Line Items]
Total	858.9	832.7
Gross Unrealized Gains	5.2	1.4
Gross Unrealized Losses	0.7	4.8
Fair Value	863.4	829.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	2,246.7	2,087.8
Gross Unrealized Gains	40.4	11.9
Gross Unrealized Losses	0.7	3.5
Fair Value	2,286.4	2,096.2
Other Asset-Backed
Investment Holdings [Line Items]
Total	2,983.8	2,678.9
Gross Unrealized Gains	11.6	1.7
Gross Unrealized Losses	5.8	22.9
Fair Value	2,989.6	2,657.7
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	783.6	587.4
Gross Unrealized Gains	28.8	4.0
Gross Unrealized Losses	0.9	4.2
Fair Value	811.5	587.2
Other
Investment Holdings [Line Items]
Total	13.6	15.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 13.6	$ 15.7