Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Investment Holdings [Line Items]
AMORTIZED COST	$ 37,948.5	$ 41,155.7
GROSS UNREALIZED GAINS	310.6	897.6
GROSS UNREALIZED LOSSES	248.6	31.3
FAIR VALUE	38,010.5	42,022.0
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	2,406.2	2,728.8
GROSS UNREALIZED GAINS	29.3	71.1
GROSS UNREALIZED LOSSES	9.4	0.0
FAIR VALUE	2,426.1	2,799.9
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	3,841.0	2,927.8
GROSS UNREALIZED GAINS	73.7	155.9
GROSS UNREALIZED LOSSES	38.6	0.1
FAIR VALUE	3,876.1	3,083.6
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	18,092.1	24,595.1
GROSS UNREALIZED GAINS	93.7	388.5
GROSS UNREALIZED LOSSES	110.2	26.9
FAIR VALUE	18,075.6	24,956.7
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	383.4	713.6
GROSS UNREALIZED GAINS	0.1	1.1
GROSS UNREALIZED LOSSES	9.5	0.7
FAIR VALUE	374.0	714.0
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	2,319.8	2,459.9
GROSS UNREALIZED GAINS	31.6	79.8
GROSS UNREALIZED LOSSES	9.7	0.1
FAIR VALUE	2,341.7	2,539.6
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	502.6	543.1
GROSS UNREALIZED GAINS	3.9	10.0
GROSS UNREALIZED LOSSES	0.9	0.0
FAIR VALUE	505.6	553.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	3,052.4	2,281.7
GROSS UNREALIZED GAINS	28.1	64.7
GROSS UNREALIZED LOSSES	35.4	0.6
FAIR VALUE	3,045.1	2,345.8
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	5,962.0	3,953.5
GROSS UNREALIZED GAINS	11.3	46.8
GROSS UNREALIZED LOSSES	31.7	2.8
FAIR VALUE	5,941.6	3,997.5
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	1,389.0	952.2
GROSS UNREALIZED GAINS	38.9	79.7
GROSS UNREALIZED LOSSES	3.2	0.1
FAIR VALUE	$ 1,424.7	$ 1,031.8