Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Investment Holdings [Line Items]
AMORTIZED COST	$ 27,760.0	$ 37,948.5
GROSS UNREALIZED GAINS	7.7	310.6
GROSS UNREALIZED LOSSES	1,067.8	248.6
FAIR VALUE	26,699.9	38,010.5
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	2,837.7	2,406.2
GROSS UNREALIZED GAINS	2.5	29.3
GROSS UNREALIZED LOSSES	92.8	9.4
FAIR VALUE	2,747.4	2,426.1
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	817.8	3,841.0
GROSS UNREALIZED GAINS	0.0	73.7
GROSS UNREALIZED LOSSES	30.2	38.6
FAIR VALUE	787.6	3,876.1
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	11,892.5	18,092.1
GROSS UNREALIZED GAINS	4.3	93.7
GROSS UNREALIZED LOSSES	351.6	110.2
FAIR VALUE	11,545.2	18,075.6
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	387.6	383.4
GROSS UNREALIZED GAINS	0.0	0.1
GROSS UNREALIZED LOSSES	27.6	9.5
FAIR VALUE	360.0	374.0
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	1,774.3	2,319.8
GROSS UNREALIZED GAINS	0.2	31.6
GROSS UNREALIZED LOSSES	26.9	9.7
FAIR VALUE	1,747.6	2,341.7
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	403.1	502.6
GROSS UNREALIZED GAINS	0.3	3.9
GROSS UNREALIZED LOSSES	14.7	0.9
FAIR VALUE	388.7	505.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	2,645.8	3,052.4
GROSS UNREALIZED GAINS	0.3	28.1
GROSS UNREALIZED LOSSES	166.7	35.4
FAIR VALUE	2,479.4	3,045.1
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	5,544.3	5,962.0
GROSS UNREALIZED GAINS	0.0	11.3
GROSS UNREALIZED LOSSES	288.1	31.7
FAIR VALUE	5,256.2	5,941.6
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	1,456.9	1,389.0
GROSS UNREALIZED GAINS	0.1	38.9
GROSS UNREALIZED LOSSES	69.2	3.2
FAIR VALUE	$ 1,387.8	$ 1,424.7