Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Available for Sale - Amortized Cost
Amortized cost, one year or less	$ 7,508.1
Amortized cost, one to five years	18,696.4
Amortized cost, five to ten years	12,334.0
Amortized cost, over ten years	2,617.2
AMORTIZED COST	41,155.7	$ 38,722.2
Available for Sale - Fair Value
Fair value, one year or less	7,598.7
Fair value, one to five years	19,076.1
Fair value, five to ten years	12,673.3
Fair value, over ten years	2,673.9
Total	42,022.0	38,876.3
Held to Maturity - Amortized Cost
Amortized cost, one year or less	11,593.6
Amortized cost, one to five years	5,607.2
Amortized cost, five to ten years	473.3
Amortized cost, over ten years	117.0
AMORTIZED COST	17,791.1	12,284.5
Held to Maturity - Fair Value
Fair value, one year or less	11,603.3
Fair value, one to five years	5,660.8
Fair value, five to ten years	468.1
Fair value, over ten years	65.2
FAIR VALUE	17,797.4	12,249.3
U.S. Government
Available for Sale - Amortized Cost
Amortized cost, one year or less	300.9
Amortized cost, one to five years	1,732.8
Amortized cost, five to ten years	695.1
Amortized cost, over ten years	0.0
AMORTIZED COST	2,728.8	4,527.5
Available for Sale - Fair Value
Fair value, one year or less	303.2
Fair value, one to five years	1,767.0
Fair value, five to ten years	729.7
Fair value, over ten years	0.0
Total	2,799.9	4,549.1
Held to Maturity - Amortized Cost
Amortized cost, one year or less	90.0
Amortized cost, one to five years	0.0
Amortized cost, five to ten years	0.0
Amortized cost, over ten years	0.0
AMORTIZED COST	90.0	138.8
Held to Maturity - Fair Value
Fair value, one year or less	90.0
Fair value, one to five years	0.0
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
FAIR VALUE	90.0	138.8
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
Amortized cost, one year or less	7.9
Amortized cost, one to five years	252.1
Amortized cost, five to ten years	2,578.4
Amortized cost, over ten years	89.4
AMORTIZED COST	2,927.8	1,604.0
Available for Sale - Fair Value
Fair value, one year or less	8.0
Fair value, one to five years	266.8
Fair value, five to ten years	2,718.4
Fair value, over ten years	90.4
Total	3,083.6	1,615.3
Held to Maturity - Amortized Cost
Amortized cost, one year or less	1.4
Amortized cost, one to five years	0.7
Amortized cost, five to ten years	0.0
Amortized cost, over ten years	0.0
AMORTIZED COST	2.1	10.1
Held to Maturity - Fair Value
Fair value, one year or less	1.4
Fair value, one to five years	0.8
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
FAIR VALUE	2.2	10.3
Government Sponsored Agency
Available for Sale - Amortized Cost
Amortized cost, one year or less	5,540.0
Amortized cost, one to five years	8,942.2
Amortized cost, five to ten years	7,682.1
Amortized cost, over ten years	2,430.8
AMORTIZED COST	24,595.1	23,247.5
Available for Sale - Fair Value
Fair value, one year or less	5,613.6
Fair value, one to five years	9,063.0
Fair value, five to ten years	7,793.6
Fair value, over ten years	2,486.5
Total	24,956.7	23,271.2
Held to Maturity - Amortized Cost
Amortized cost, one year or less	0.5
Amortized cost, one to five years	1.3
Amortized cost, five to ten years	0.8
Amortized cost, over ten years	0.4
AMORTIZED COST	3.0	4.1
Held to Maturity - Fair Value
Fair value, one year or less	0.5
Fair value, one to five years	1.4
Fair value, five to ten years	0.9
Fair value, over ten years	0.5
FAIR VALUE	3.3	4.3
Non-U.S. Government
Available for Sale - Amortized Cost
Amortized cost, one year or less	414.3
Amortized cost, one to five years	40.6
Amortized cost, five to ten years	258.7
Amortized cost, over ten years	0.0
AMORTIZED COST	713.6
Available for Sale - Fair Value
Fair value, one year or less	414.6
Fair value, one to five years	40.8
Fair value, five to ten years	258.6
Fair value, over ten years	0.0
Total	714.0
Held to Maturity - Amortized Cost
Amortized cost, one year or less	8,065.4
Amortized cost, one to five years	271.2
Amortized cost, five to ten years	0.0
Amortized cost, over ten years	0.0
AMORTIZED COST	8,336.6	4,076.0
Held to Maturity - Fair Value
Fair value, one year or less	8,065.5
Fair value, one to five years	278.2
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
FAIR VALUE	8,343.7	4,078.8
Corporate Debt
Available for Sale - Amortized Cost
Amortized cost, one year or less	443.5
Amortized cost, one to five years	2,016.4
Amortized cost, five to ten years	0.0
Amortized cost, over ten years	0.0
AMORTIZED COST	2,459.9	2,378.9
Available for Sale - Fair Value
Fair value, one year or less	448.6
Fair value, one to five years	2,091.0
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
Total	2,539.6	2,402.7
Held to Maturity - Amortized Cost
Amortized cost, one year or less	126.2
Amortized cost, one to five years	461.8
Amortized cost, five to ten years	0.0
Amortized cost, over ten years	0.0
AMORTIZED COST	588.0	405.1
Held to Maturity - Fair Value
Fair value, one year or less	126.3
Fair value, one to five years	468.1
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
FAIR VALUE	594.4	406.2
Covered Bonds
Available for Sale - Amortized Cost
Amortized cost, one year or less	108.2
Amortized cost, one to five years	434.9
Amortized cost, five to ten years	0.0
Amortized cost, over ten years	0.0
AMORTIZED COST	543.1	766.3
Available for Sale - Fair Value
Fair value, one year or less	108.6
Fair value, one to five years	444.5
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
Total	553.1	769.9
Held to Maturity - Amortized Cost
Amortized cost, one year or less	1,283.7
Amortized cost, one to five years	1,836.3
Amortized cost, five to ten years	64.6
Amortized cost, over ten years	0.0
AMORTIZED COST	3,184.6	3,006.7
Held to Maturity - Fair Value
Fair value, one year or less	1,289.5
Fair value, one to five years	1,854.7
Fair value, five to ten years	64.7
Fair value, over ten years	0.0
FAIR VALUE	3,208.9	3,020.4
Certificates of Deposit
Held to Maturity - Amortized Cost
Amortized cost, one year or less	807.2
Amortized cost, one to five years	0.0
Amortized cost, five to ten years	0.0
Amortized cost, over ten years	0.0
AMORTIZED COST	807.2	262.9
Held to Maturity - Fair Value
Fair value, one year or less	807.2
Fair value, one to five years	0.0
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
FAIR VALUE	807.2	262.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
Amortized cost, one year or less	163.9
Amortized cost, one to five years	1,960.8
Amortized cost, five to ten years	157.0
Amortized cost, over ten years	0.0
AMORTIZED COST	2,281.7	2,091.3
Available for Sale - Fair Value
Fair value, one year or less	164.1
Fair value, one to five years	2,024.0
Fair value, five to ten years	157.7
Fair value, over ten years	0.0
Total	2,345.8	2,127.6
Held to Maturity - Amortized Cost
Amortized cost, one year or less	943.2
Amortized cost, one to five years	2,354.8
Amortized cost, five to ten years	350.0
Amortized cost, over ten years	0.0
AMORTIZED COST	3,648.0	3,285.4
Held to Maturity - Fair Value
Fair value, one year or less	947.2
Fair value, one to five years	2,393.4
Fair value, five to ten years	350.0
Fair value, over ten years	0.0
FAIR VALUE	3,690.6	3,305.0
Other Asset-Backed
Available for Sale - Amortized Cost
Amortized cost, one year or less	517.4
Amortized cost, one to five years	2,903.1
Amortized cost, five to ten years	436.0
Amortized cost, over ten years	97.0
AMORTIZED COST	3,953.5	3,324.5
Available for Sale - Fair Value
Fair value, one year or less	525.9
Fair value, one to five years	2,937.8
Fair value, five to ten years	436.8
Fair value, over ten years	97.0
Total	3,997.5	3,330.5
Held to Maturity - Amortized Cost
Amortized cost, one year or less	239.4
Amortized cost, one to five years	433.4
Amortized cost, five to ten years	4.2
Amortized cost, over ten years	0.0
AMORTIZED COST	677.0	804.3
Held to Maturity - Fair Value
Fair value, one year or less	239.7
Fair value, one to five years	433.9
Fair value, five to ten years	4.3
Fair value, over ten years	0.0
FAIR VALUE	677.9	804.7
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
Amortized cost, one year or less	12.0
Amortized cost, one to five years	413.5
Amortized cost, five to ten years	526.7
Amortized cost, over ten years	0.0
AMORTIZED COST	952.2	769.9
Available for Sale - Fair Value
Fair value, one year or less	12.1
Fair value, one to five years	441.2
Fair value, five to ten years	578.5
Fair value, over ten years	0.0
Total	1,031.8	797.7
Other
Available for Sale - Amortized Cost
AMORTIZED COST		9.0
Available for Sale - Fair Value
Total		9.0
Held to Maturity - Amortized Cost
Amortized cost, one year or less	36.6
Amortized cost, one to five years	247.7
Amortized cost, five to ten years	53.7
Amortized cost, over ten years	116.6
AMORTIZED COST	454.6	291.1
Held to Maturity - Fair Value
Fair value, one year or less	36.0
Fair value, one to five years	230.3
Fair value, five to ten years	48.2
Fair value, over ten years	64.7
FAIR VALUE	$ 379.2	$ 217.9