Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Investment Holdings [Line Items]
AMORTIZED COST	$ 36,852.2	$ 37,948.5
GROSS UNREALIZED GAINS	20.9	310.6
GROSS UNREALIZED LOSSES	1,855.2	248.6
FAIR VALUE	35,017.9	38,010.5
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	2,645.2	2,406.2
GROSS UNREALIZED GAINS	4.4	29.3
GROSS UNREALIZED LOSSES	61.2	9.4
FAIR VALUE	2,588.4	2,426.1
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	3,883.1	3,841.0
GROSS UNREALIZED GAINS	0.2	73.7
GROSS UNREALIZED LOSSES	430.0	38.6
FAIR VALUE	3,453.3	3,876.1
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	17,151.3	18,092.1
GROSS UNREALIZED GAINS	13.5	93.7
GROSS UNREALIZED LOSSES	751.6	110.2
FAIR VALUE	16,413.2	18,075.6
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	396.2	383.4
GROSS UNREALIZED GAINS	0.0	0.1
GROSS UNREALIZED LOSSES	28.9	9.5
FAIR VALUE	367.3	374.0
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	2,163.4	2,319.8
GROSS UNREALIZED GAINS	0.8	31.6
GROSS UNREALIZED LOSSES	76.3	9.7
FAIR VALUE	2,087.9	2,341.7
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	541.6	502.6
GROSS UNREALIZED GAINS	0.3	3.9
GROSS UNREALIZED LOSSES	12.9	0.9
FAIR VALUE	529.0	505.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	2,939.6	3,052.4
GROSS UNREALIZED GAINS	1.7	28.1
GROSS UNREALIZED LOSSES	147.5	35.4
FAIR VALUE	2,793.8	3,045.1
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	5,663.2	5,962.0
GROSS UNREALIZED GAINS	0.0	11.3
GROSS UNREALIZED LOSSES	271.7	31.7
FAIR VALUE	5,391.5	5,941.6
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	1,468.6	1,389.0
GROSS UNREALIZED GAINS	0.0	38.9
GROSS UNREALIZED LOSSES	75.1	3.2
FAIR VALUE	$ 1,393.5	$ 1,424.7