Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Investment Holdings [Line Items]
AMORTIZED COST	$ 37,973.7	$ 37,948.5
GROSS UNREALIZED GAINS	51.6	310.6
GROSS UNREALIZED LOSSES	1,164.4	248.6
FAIR VALUE	36,860.9	38,010.5
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	2,581.5	2,406.2
GROSS UNREALIZED GAINS	6.8	29.3
GROSS UNREALIZED LOSSES	41.3	9.4
FAIR VALUE	2,547.0	2,426.1
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	3,899.7	3,841.0
GROSS UNREALIZED GAINS	4.7	73.7
GROSS UNREALIZED LOSSES	256.6	38.6
FAIR VALUE	3,647.8	3,876.1
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	17,383.9	18,092.1
GROSS UNREALIZED GAINS	19.3	93.7
GROSS UNREALIZED LOSSES	475.9	110.2
FAIR VALUE	16,927.3	18,075.6
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	645.8	383.4
GROSS UNREALIZED GAINS	0.0	0.1
GROSS UNREALIZED LOSSES	24.4	9.5
FAIR VALUE	621.4	374.0
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	2,337.9	2,319.8
GROSS UNREALIZED GAINS	5.5	31.6
GROSS UNREALIZED LOSSES	51.9	9.7
FAIR VALUE	2,291.5	2,341.7
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	580.1	502.6
GROSS UNREALIZED GAINS	1.3	3.9
GROSS UNREALIZED LOSSES	7.9	0.9
FAIR VALUE	573.5	505.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	2,946.3	3,052.4
GROSS UNREALIZED GAINS	8.0	28.1
GROSS UNREALIZED LOSSES	114.0	35.4
FAIR VALUE	2,840.3	3,045.1
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	6,099.2	5,962.0
GROSS UNREALIZED GAINS	0.7	11.3
GROSS UNREALIZED LOSSES	157.5	31.7
FAIR VALUE	5,942.4	5,941.6
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	1,499.3	1,389.0
GROSS UNREALIZED GAINS	5.3	38.9
GROSS UNREALIZED LOSSES	34.9	3.2
FAIR VALUE	$ 1,469.7	$ 1,424.7