Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Available for Sale - Cost
Due in One Year or Less	$ 9,742.4
Due After One Year Through Five Years	18,342.9
Due After Five Years Through Ten Years	8,752.3
Due After Ten Years	2,287.1
Total	39,124.7	$ 38,722.2
Available for Sale - Value
Due in One Year or Less	9,848.4
Due After One Year Through Five Years	18,733.3
Due After Five Years Through Ten Years	9,038.5
Due After Ten Years	2,348.7
Total	39,968.9	38,876.3
Held to Maturity - Amortized Cost
Due in One Year or Less	9,128.0
Due After One Year Through Five Years	5,884.3
Due After Five Years Through Ten Years	118.2
Due After Ten Years	111.4
Total	15,241.9	12,284.5
Held to Maturity - Fair Value
Due in One Year or Less	9,138.2
Due After One Year Through Five Years	5,920.6
Due After Five Years Through Ten Years	109.9
Due After Ten Years	54.7
Total	15,223.4	12,249.3
U.S. Government
Available for Sale - Cost
Due in One Year or Less	1,804.3
Due After One Year Through Five Years	1,732.4
Due After Five Years Through Ten Years	552.3
Due After Ten Years	0.0
Total	4,089.0	4,527.5
Available for Sale - Value
Due in One Year or Less	1,811.7
Due After One Year Through Five Years	1,768.5
Due After Five Years Through Ten Years	596.2
Due After Ten Years	0.0
Total	4,176.4	4,549.1
Held to Maturity - Amortized Cost
Due in One Year or Less	98.0
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	98.0	138.8
Held to Maturity - Fair Value
Due in One Year or Less	98.0
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	98.0	138.8
Obligations of States and Political Subdivisions
Available for Sale - Cost
Due in One Year or Less	21.3
Due After One Year Through Five Years	143.3
Due After Five Years Through Ten Years	1,763.0
Due After Ten Years	0.0
Total	1,927.6	1,604.0
Available for Sale - Value
Due in One Year or Less	21.3
Due After One Year Through Five Years	150.7
Due After Five Years Through Ten Years	1,853.9
Due After Ten Years	0.0
Total	2,025.9	1,615.3
Held to Maturity - Amortized Cost
Due in One Year or Less	6.0
Due After One Year Through Five Years	1.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	7.0	10.1
Held to Maturity - Fair Value
Due in One Year or Less	6.1
Due After One Year Through Five Years	1.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	7.1	10.3
Government Sponsored Agency
Available for Sale - Cost
Due in One Year or Less	5,798.0
Due After One Year Through Five Years	9,595.9
Due After Five Years Through Ten Years	5,357.1
Due After Ten Years	2,214.8
Total	22,965.8	23,247.5
Available for Sale - Value
Due in One Year or Less	5,888.8
Due After One Year Through Five Years	9,766.5
Due After Five Years Through Ten Years	5,474.3
Due After Ten Years	2,275.8
Total	23,405.4	23,271.2
Held to Maturity - Amortized Cost
Due in One Year or Less	0.5
Due After One Year Through Five Years	1.6
Due After Five Years Through Ten Years	1.0
Due After Ten Years	0.5
Total	3.6	4.1
Held to Maturity - Fair Value
Due in One Year or Less	0.6
Due After One Year Through Five Years	1.7
Due After Five Years Through Ten Years	1.1
Due After Ten Years	0.5
Total	3.9	4.3
Non-U.S. Government
Available for Sale - Cost
Due in One Year or Less	742.5
Due After One Year Through Five Years	2.1
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	744.6
Available for Sale - Value
Due in One Year or Less	742.6
Due After One Year Through Five Years	2.1
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	744.7
Held to Maturity - Amortized Cost
Due in One Year or Less	5,887.1
Due After One Year Through Five Years	247.5
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	6,134.6	4,076.0
Held to Maturity - Fair Value
Due in One Year or Less	5,889.9
Due After One Year Through Five Years	253.3
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	6,143.2	4,078.8
Corporate Debt
Available for Sale - Cost
Due in One Year or Less	447.6
Due After One Year Through Five Years	2,049.9
Due After Five Years Through Ten Years	18.4
Due After Ten Years	0.0
Total	2,515.9	2,378.9
Available for Sale - Value
Due in One Year or Less	451.3
Due After One Year Through Five Years	2,121.1
Due After Five Years Through Ten Years	18.4
Due After Ten Years	0.0
Total	2,590.8	2,402.7
Held to Maturity - Amortized Cost
Due in One Year or Less	155.0
Due After One Year Through Five Years	354.6
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	509.6	405.1
Held to Maturity - Fair Value
Due in One Year or Less	155.1
Due After One Year Through Five Years	358.4
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	513.5	406.2
Covered Bonds
Available for Sale - Cost
Due in One Year or Less	114.9
Due After One Year Through Five Years	491.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	605.9	766.3
Available for Sale - Value
Due in One Year or Less	115.1
Due After One Year Through Five Years	496.9
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	612.0	769.9
Held to Maturity - Amortized Cost
Due in One Year or Less	1,023.2
Due After One Year Through Five Years	2,174.8
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	3,198.0	3,006.7
Held to Maturity - Fair Value
Due in One Year or Less	1,027.8
Due After One Year Through Five Years	2,189.8
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	3,217.6	3,020.4
Certificates of Deposit
Available for Sale - Cost
Due in One Year or Less	383.0
Due After One Year Through Five Years	1,809.2
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	2,192.2
Available for Sale - Value
Due in One Year or Less	383.4
Due After One Year Through Five Years	1,874.3
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	2,257.7
Held to Maturity - Amortized Cost
Due in One Year or Less	1,176.3
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	1,176.3	262.9
Held to Maturity - Fair Value
Due in One Year or Less	1,176.2
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	1,176.2	262.9
Other Asset-Backed
Available for Sale - Cost
Due in One Year or Less	405.8
Due After One Year Through Five Years	2,211.5
Due After Five Years Through Ten Years	490.8
Due After Ten Years	61.6
Total	3,169.7	3,324.5
Available for Sale - Value
Due in One Year or Less	409.2
Due After One Year Through Five Years	2,229.2
Due After Five Years Through Ten Years	482.6
Due After Ten Years	61.6
Total	3,182.6	3,330.5
Held to Maturity - Amortized Cost
Due in One Year or Less	214.3
Due After One Year Through Five Years	454.7
Due After Five Years Through Ten Years	27.2
Due After Ten Years	0.0
Total	696.2	804.3
Held to Maturity - Fair Value
Due in One Year or Less	214.1
Due After One Year Through Five Years	452.5
Due After Five Years Through Ten Years	27.2
Due After Ten Years	0.0
Total	693.8	804.7
Commercial Mortgage-Backed
Available for Sale - Cost
Due in One Year or Less	17.0
Due After One Year Through Five Years	307.6
Due After Five Years Through Ten Years	570.7
Due After Ten Years	10.7
Total	906.0	769.9
Available for Sale - Value
Due in One Year or Less	17.0
Due After One Year Through Five Years	324.0
Due After Five Years Through Ten Years	613.1
Due After Ten Years	11.3
Total	965.4	797.7
Other
Available for Sale - Cost
Due in One Year or Less	8.0
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	8.0	9.0
Available for Sale - Value
Due in One Year or Less	8.0
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	8.0	9.0
Held to Maturity - Amortized Cost
Due in One Year or Less	17.7
Due After One Year Through Five Years	241.6
Due After Five Years Through Ten Years	49.1
Due After Ten Years	110.9
Total	419.3	291.1
Held to Maturity - Fair Value
Due in One Year or Less	17.2
Due After One Year Through Five Years	214.7
Due After Five Years Through Ten Years	40.9
Due After Ten Years	54.2
Total	327.0	217.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Cost
Total	2,192.2	2,091.3
Available for Sale - Value
Total	2,257.7	2,127.6
Held to Maturity - Amortized Cost
Due in One Year or Less	549.9
Due After One Year Through Five Years	2,408.5
Due After Five Years Through Ten Years	40.9
Due After Ten Years	0.0
Total	2,999.3	3,285.4
Held to Maturity - Fair Value
Due in One Year or Less	553.2
Due After One Year Through Five Years	2,449.2
Due After Five Years Through Ten Years	40.7
Due After Ten Years	0.0
Total	$ 3,043.1	$ 3,305.0