Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Investment Holdings [Line Items]
Total Fair Value	$ 36,464.6	$ 41,155.7
GROSS UNREALIZED GAINS	511.6	897.6
GROSS UNREALIZED LOSSES	112.2	31.3
FAIR VALUE	36,864.0	42,022.0
U.S. Government
Investment Holdings [Line Items]
Total Fair Value	2,519.3	2,728.8
GROSS UNREALIZED GAINS	46.6	71.1
GROSS UNREALIZED LOSSES	6.9	0.0
FAIR VALUE	2,559.0	2,799.9
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total Fair Value	3,398.6	2,927.8
GROSS UNREALIZED GAINS	108.8	155.9
GROSS UNREALIZED LOSSES	18.5	0.1
FAIR VALUE	3,488.9	3,083.6
Government Sponsored Agency
Investment Holdings [Line Items]
Total Fair Value	17,558.7	24,595.1
GROSS UNREALIZED GAINS	151.8	388.5
GROSS UNREALIZED LOSSES	64.7	26.9
FAIR VALUE	17,645.8	24,956.7
Non-U.S. Government
Investment Holdings [Line Items]
Total Fair Value	628.5	713.6
GROSS UNREALIZED GAINS	0.3	1.1
GROSS UNREALIZED LOSSES	5.0	0.7
FAIR VALUE	623.8	714.0
Corporate Debt
Investment Holdings [Line Items]
Total Fair Value	2,405.7	2,459.9
GROSS UNREALIZED GAINS	59.1	79.8
GROSS UNREALIZED LOSSES	1.9	0.1
FAIR VALUE	2,462.9	2,539.6
Covered Bonds
Investment Holdings [Line Items]
Total Fair Value	528.1	543.1
GROSS UNREALIZED GAINS	7.3	10.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	535.4	553.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total Fair Value	3,023.5	2,281.7
GROSS UNREALIZED GAINS	47.8	64.7
GROSS UNREALIZED LOSSES	12.8	0.6
FAIR VALUE	3,058.5	2,345.8
Other Asset-Backed
Investment Holdings [Line Items]
Total Fair Value	5,248.6	3,953.5
GROSS UNREALIZED GAINS	31.3	46.8
GROSS UNREALIZED LOSSES	1.8	2.8
FAIR VALUE	5,278.1	3,997.5
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total Fair Value	1,153.6	952.2
GROSS UNREALIZED GAINS	58.6	79.7
GROSS UNREALIZED LOSSES	0.6	0.1
FAIR VALUE	$ 1,211.6	$ 1,031.8