Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Total	$ 39,124.7	$ 38,722.2
GROSS UNREALIZED GAINS	895.9	262.7
GROSS UNREALIZED LOSSES	51.7	108.6
FAIR VALUE	39,968.9	38,876.3
U.S. Government
Investment Holdings [Line Items]
Total	4,089.0	4,527.5
GROSS UNREALIZED GAINS	87.4	26.7
GROSS UNREALIZED LOSSES	0.0	5.1
FAIR VALUE	4,176.4	4,549.1
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	1,927.6	1,604.0
GROSS UNREALIZED GAINS	98.7	24.6
GROSS UNREALIZED LOSSES	0.4	13.3
FAIR VALUE	2,025.9	1,615.3
Government Sponsored Agency
Investment Holdings [Line Items]
Total	22,965.8	23,247.5
GROSS UNREALIZED GAINS	461.5	101.8
GROSS UNREALIZED LOSSES	21.9	78.1
FAIR VALUE	23,405.4	23,271.2
Non-U.S. Government
Investment Holdings [Line Items]
Total	744.6	3.3
GROSS UNREALIZED GAINS	0.1	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	744.7	3.3
Corporate Debt
Investment Holdings [Line Items]
Total	2,515.9	2,378.9
GROSS UNREALIZED GAINS	77.6	27.8
GROSS UNREALIZED LOSSES	2.7	4.0
FAIR VALUE	2,590.8	2,402.7
Covered Bonds
Investment Holdings [Line Items]
Total	605.9	766.3
GROSS UNREALIZED GAINS	6.4	4.4
GROSS UNREALIZED LOSSES	0.3	0.8
FAIR VALUE	612.0	769.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	2,192.2	2,091.3
GROSS UNREALIZED GAINS	65.8	37.4
GROSS UNREALIZED LOSSES	0.3	1.1
FAIR VALUE	2,257.7	2,127.6
Other Asset-Backed
Investment Holdings [Line Items]
Total	3,169.7	3,324.5
GROSS UNREALIZED GAINS	37.3	11.3
GROSS UNREALIZED LOSSES	24.4	5.3
FAIR VALUE	3,182.6	3,330.5
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	906.0	769.9
GROSS UNREALIZED GAINS	61.1	28.7
GROSS UNREALIZED LOSSES	1.7	0.9
FAIR VALUE	965.4	797.7
Other
Investment Holdings [Line Items]
Total	8.0	9.0
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	$ 8.0	$ 9.0