Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
AMORTIZED COST	$ 41,155.7	$ 38,722.2
GROSS UNREALIZED GAINS	897.6	262.7
GROSS UNREALIZED LOSSES	31.3	108.6
FAIR VALUE	42,022.0	38,876.3
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	2,728.8	4,527.5
GROSS UNREALIZED GAINS	71.1	26.7
GROSS UNREALIZED LOSSES	0.0	5.1
FAIR VALUE	2,799.9	4,549.1
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	2,927.8	1,604.0
GROSS UNREALIZED GAINS	155.9	24.6
GROSS UNREALIZED LOSSES	0.1	13.3
FAIR VALUE	3,083.6	1,615.3
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	24,595.1	23,247.5
GROSS UNREALIZED GAINS	388.5	101.8
GROSS UNREALIZED LOSSES	26.9	78.1
FAIR VALUE	24,956.7	23,271.2
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	713.6	3.3
GROSS UNREALIZED GAINS	1.1	0.0
GROSS UNREALIZED LOSSES	0.7	0.0
FAIR VALUE	714.0	3.3
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	2,459.9	2,378.9
GROSS UNREALIZED GAINS	79.8	27.8
GROSS UNREALIZED LOSSES	0.1	4.0
FAIR VALUE	2,539.6	2,402.7
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	543.1	766.3
GROSS UNREALIZED GAINS	10.0	4.4
GROSS UNREALIZED LOSSES	0.0	0.8
FAIR VALUE	553.1	769.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	2,281.7	2,091.3
GROSS UNREALIZED GAINS	64.7	37.4
GROSS UNREALIZED LOSSES	0.6	1.1
FAIR VALUE	2,345.8	2,127.6
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	3,953.5	3,324.5
GROSS UNREALIZED GAINS	46.8	11.3
GROSS UNREALIZED LOSSES	2.8	5.3
FAIR VALUE	3,997.5	3,330.5
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	952.2	769.9
GROSS UNREALIZED GAINS	79.7	28.7
GROSS UNREALIZED LOSSES	0.1	0.9
FAIR VALUE	$ 1,031.8	797.7
Other
Investment Holdings [Line Items]
AMORTIZED COST		9.0
GROSS UNREALIZED GAINS		0.0
GROSS UNREALIZED LOSSES		0.0
FAIR VALUE		$ 9.0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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