Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Available for Sale - Cost
Due in One Year or Less	$ 9,548.5
Due After One Year Through Five Years	18,478.5
Due After Five Years Through Ten Years	8,799.2
Due After Ten Years	2,199.1
Total	39,025.3	$ 38,722.2
Available for Sale - Value
Due in One Year or Less	9,634.1
Due After One Year Through Five Years	18,676.5
Due After Five Years Through Ten Years	8,914.7
Due After Ten Years	2,216.4
Total	39,441.7	38,876.3
Held to Maturity - Amortized Cost
Due in One Year or Less	6,163.4
Due After One Year Through Five Years	6,197.0
Due After Five Years Through Ten Years	169.4
Due After Ten Years	105.9
Total	12,635.7	12,284.5
Held to Maturity - Fair Value
Due in One Year or Less	6,165.9
Due After One Year Through Five Years	6,191.4
Due After Five Years Through Ten Years	150.9
Due After Ten Years	24.8
Total	12,533.0	12,249.3
U.S. Government
Available for Sale - Cost
Due in One Year or Less	2,204.6
Due After One Year Through Five Years	1,724.7
Due After Five Years Through Ten Years	552.4
Due After Ten Years	0.0
Total	4,481.7	4,527.5
Available for Sale - Value
Due in One Year or Less	2,221.5
Due After One Year Through Five Years	1,768.4
Due After Five Years Through Ten Years	594.2
Due After Ten Years	0.0
Total	4,584.1	4,549.1
Held to Maturity - Amortized Cost
Due in One Year or Less	104.9
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	104.9	138.8
Held to Maturity - Fair Value
Due in One Year or Less	105.0
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	105.0	138.8
Obligations of States and Political Subdivisions
Available for Sale - Cost
Due in One Year or Less	35.9
Due After One Year Through Five Years	87.5
Due After Five Years Through Ten Years	1,601.8
Due After Ten Years	0.0
Total	1,725.2	1,604.0
Available for Sale - Value
Due in One Year or Less	35.9
Due After One Year Through Five Years	91.3
Due After Five Years Through Ten Years	1,649.1
Due After Ten Years	0.0
Total	1,776.3	1,615.3
Held to Maturity - Amortized Cost
Due in One Year or Less	6.0
Due After One Year Through Five Years	0.9
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	6.9	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	5,825.0
Due After One Year Through Five Years	10,097.6
Due After Five Years Through Ten Years	5,524.0
Due After Ten Years	2,164.4
Total	23,611.0	23,247.5
Available for Sale - Value
Due in One Year or Less	5,895.7
Due After One Year Through Five Years	10,225.5
Due After Five Years Through Ten Years	5,565.7
Due After Ten Years	2,183.1
Total	23,870.0	23,271.2
Held to Maturity - Amortized Cost
Due in One Year or Less	0.6
Due After One Year Through Five Years	1.7
Due After Five Years Through Ten Years	1.0
Due After Ten Years	0.6
Total	3.9	4.1
Held to Maturity - Fair Value
Due in One Year or Less	0.7
Due After One Year Through Five Years	1.8
Due After Five Years Through Ten Years	1.1
Due After Ten Years	0.6
Total	4.2	4.3
Non-U.S. Government
Available for Sale - Cost
Due in One Year or Less	3.3
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	3.3	3.3
Available for Sale - Value
Due in One Year or Less	3.4
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	3.4	3.3
Held to Maturity - Amortized Cost
Due in One Year or Less	3,481.7
Due After One Year Through Five Years	214.3
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	3,696.0	3,006.7
Held to Maturity - Fair Value
Due in One Year or Less	3,483.9
Due After One Year Through Five Years	219.3
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	3,703.2	3,020.4
Corporate Debt
Available for Sale - Cost
Due in One Year or Less	366.4
Due After One Year Through Five Years	2,036.4
Due After Five Years Through Ten Years	5.8
Due After Ten Years	0.0
Total	2,408.6	2,378.9
Available for Sale - Value
Due in One Year or Less	365.8
Due After One Year Through Five Years	2,064.4
Due After Five Years Through Ten Years	5.8
Due After Ten Years	0.0
Total	2,436.0	2,402.7
Held to Maturity - Amortized Cost
Due in One Year or Less	96.0
Due After One Year Through Five Years	424.9
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	520.9	4,076.0
Held to Maturity - Fair Value
Due in One Year or Less	94.2
Due After One Year Through Five Years	419.4
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	513.6	4,078.8
Covered Bonds
Available for Sale - Cost
Due in One Year or Less	108.3
Due After One Year Through Five Years	442.2
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	550.5	766.3
Available for Sale - Value
Due in One Year or Less	108.6
Due After One Year Through Five Years	444.8
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	553.4	769.9
Held to Maturity - Amortized Cost
Due in One Year or Less	753.0
Due After One Year Through Five Years	2,363.9
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	3,116.9	405.1
Held to Maturity - Fair Value
Due in One Year or Less	755.2
Due After One Year Through Five Years	2,363.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	3,118.2	406.2
Certificates of Deposit
Available for Sale - Cost
Due in One Year or Less	327.6
Due After One Year Through Five Years	1,669.9
Due After Five Years Through Ten Years	50.0
Due After Ten Years	0.0
Total	2,047.5
Available for Sale - Value
Due in One Year or Less	327.8
Due After One Year Through Five Years	1,727.8
Due After Five Years Through Ten Years	49.9
Due After Ten Years	0.0
Total	2,105.5
Held to Maturity - Amortized Cost
Due in One Year or Less	1,041.9
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	1,041.9	262.9
Held to Maturity - Fair Value
Due in One Year or Less	1,041.9
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	1,041.9	262.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Cost
Total	2,047.5	2,091.3
Available for Sale - Value
Total	2,105.5	2,127.6
Held to Maturity - Amortized Cost
Due in One Year or Less	418.5
Due After One Year Through Five Years	2,600.9
Due After Five Years Through Ten Years	40.6
Due After Ten Years	0.0
Total	3,060.0	3,285.4
Held to Maturity - Fair Value
Due in One Year or Less	419.4
Due After One Year Through Five Years	2,632.6
Due After Five Years Through Ten Years	40.3
Due After Ten Years	0.0
Total	3,092.3	3,305.0
Other Asset-Backed
Available for Sale - Cost
Due in One Year or Less	643.0
Due After One Year Through Five Years	2,164.5
Due After Five Years Through Ten Years	464.4
Due After Ten Years	13.0
Total	3,284.9	3,324.5
Available for Sale - Value
Due in One Year or Less	641.2
Due After One Year Through Five Years	2,096.3
Due After Five Years Through Ten Years	432.5
Due After Ten Years	12.1
Total	3,182.1	3,330.5
Held to Maturity - Amortized Cost
Due in One Year or Less	252.3
Due After One Year Through Five Years	449.2
Due After Five Years Through Ten Years	81.8
Due After Ten Years	0.0
Total	783.3	804.3
Held to Maturity - Fair Value
Due in One Year or Less	251.5
Due After One Year Through Five Years	442.7
Due After Five Years Through Ten Years	81.0
Due After Ten Years	0.0
Total	775.2	804.7
Commercial Mortgage-Backed
Available for Sale - Cost
Due in One Year or Less	26.1
Due After One Year Through Five Years	255.7
Due After Five Years Through Ten Years	600.8
Due After Ten Years	21.7
Total	904.3	769.9
Available for Sale - Value
Due in One Year or Less	25.9
Due After One Year Through Five Years	258.0
Due After Five Years Through Ten Years	617.5
Due After Ten Years	21.2
Total	922.6	797.7
Other
Available for Sale - Cost
Due in One Year or Less	8.3
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	8.3	9.0
Available for Sale - Value
Due in One Year or Less	8.3
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	8.3	9.0
Held to Maturity - Amortized Cost
Due in One Year or Less	8.5
Due After One Year Through Five Years	141.2
Due After Five Years Through Ten Years	46.0
Due After Ten Years	105.3
Total	301.0	291.1
Held to Maturity - Fair Value
Due in One Year or Less	8.0
Due After One Year Through Five Years	111.6
Due After Five Years Through Ten Years	28.5
Due After Ten Years	24.2
Total	$ 172.3	$ 217.9