Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Total	$ 41,659.2	$ 38,722.2
GROSS UNREALIZED GAINS	926.8	262.7
GROSS UNREALIZED LOSSES	35.0	108.6
FAIR VALUE	42,551.0	38,876.3
U.S. Government
Investment Holdings [Line Items]
Total	4,083.9	4,527.5
GROSS UNREALIZED GAINS	81.3	26.7
GROSS UNREALIZED LOSSES	0.0	5.1
FAIR VALUE	4,165.2	4,549.1
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	2,465.2	1,604.0
GROSS UNREALIZED GAINS	136.3	24.6
GROSS UNREALIZED LOSSES	1.0	13.3
FAIR VALUE	2,600.5	1,615.3
Government Sponsored Agency
Investment Holdings [Line Items]
Total	24,162.6	23,247.5
GROSS UNREALIZED GAINS	427.4	101.8
GROSS UNREALIZED LOSSES	21.2	78.1
FAIR VALUE	24,568.8	23,271.2
Non-U.S. Government
Investment Holdings [Line Items]
Total	936.8	3.3
GROSS UNREALIZED GAINS	0.9	0.0
GROSS UNREALIZED LOSSES	0.1	0.0
FAIR VALUE	937.6	3.3
Corporate Debt
Investment Holdings [Line Items]
Total	2,548.7	2,378.9
GROSS UNREALIZED GAINS	80.9	27.8
GROSS UNREALIZED LOSSES	2.2	4.0
FAIR VALUE	2,627.4	2,402.7
Covered Bonds
Investment Holdings [Line Items]
Total	508.5	766.3
GROSS UNREALIZED GAINS	8.3	4.4
GROSS UNREALIZED LOSSES	0.0	0.8
FAIR VALUE	516.8	769.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	2,486.4	2,091.3
GROSS UNREALIZED GAINS	66.7	37.4
GROSS UNREALIZED LOSSES	0.3	1.1
FAIR VALUE	2,552.8	2,127.6
Other Asset-Backed
Investment Holdings [Line Items]
Total	3,522.3	3,324.5
GROSS UNREALIZED GAINS	46.6	11.3
GROSS UNREALIZED LOSSES	9.5	5.3
FAIR VALUE	3,559.4	3,330.5
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	937.5	769.9
GROSS UNREALIZED GAINS	78.4	28.7
GROSS UNREALIZED LOSSES	0.7	0.9
FAIR VALUE	1,015.2	797.7
Other
Investment Holdings [Line Items]
Total	7.3	9.0
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	$ 7.3	$ 9.0