Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Investment Holdings [Line Items]
Total	$ 42,508.6	$ 41,155.7
GROSS UNREALIZED GAINS	627.7	897.6
GROSS UNREALIZED LOSSES	257.7	31.3
FAIR VALUE	42,878.6	42,022.0
U.S. Government
Investment Holdings [Line Items]
Total	2,521.1	2,728.8
GROSS UNREALIZED GAINS	50.6	71.1
GROSS UNREALIZED LOSSES	10.7	0.0
FAIR VALUE	2,561.0	2,799.9
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	3,249.8	2,927.8
GROSS UNREALIZED GAINS	82.5	155.9
GROSS UNREALIZED LOSSES	52.5	0.1
FAIR VALUE	3,279.8	3,083.6
Government Sponsored Agency
Investment Holdings [Line Items]
Total	24,535.5	24,595.1
GROSS UNREALIZED GAINS	275.4	388.5
GROSS UNREALIZED LOSSES	160.2	26.9
FAIR VALUE	24,650.7	24,956.7
Non-U.S. Government
Investment Holdings [Line Items]
Total	564.3	713.6
GROSS UNREALIZED GAINS	0.1	1.1
GROSS UNREALIZED LOSSES	6.3	0.7
FAIR VALUE	558.1	714.0
Corporate Debt
Investment Holdings [Line Items]
Total	2,336.3	2,459.9
GROSS UNREALIZED GAINS	65.2	79.8
GROSS UNREALIZED LOSSES	2.8	0.1
FAIR VALUE	2,398.7	2,539.6
Covered Bonds
Investment Holdings [Line Items]
Total	535.8	543.1
GROSS UNREALIZED GAINS	8.3	10.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	544.1	553.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	2,968.8	2,281.7
GROSS UNREALIZED GAINS	54.0	64.7
GROSS UNREALIZED LOSSES	19.4	0.6
FAIR VALUE	3,003.4	2,345.8
Other Asset-Backed
Investment Holdings [Line Items]
Total	4,749.5	3,953.5
GROSS UNREALIZED GAINS	38.0	46.8
GROSS UNREALIZED LOSSES	4.5	2.8
FAIR VALUE	4,783.0	3,997.5
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	1,047.5	952.2
GROSS UNREALIZED GAINS	53.6	79.7
GROSS UNREALIZED LOSSES	1.3	0.1
FAIR VALUE	$ 1,099.8	$ 1,031.8