Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Available for Sale - Cost
Due in One Year or Less	$ 9,144.1	$ 9,206.2
Due After One Year Through Five Years	19,139.3	21,012.7
Due After Five Years Through Ten Years	8,399.8	5,774.1
Due After Ten Years	1,981.8	1,048.8
Total	38,665.0	37,041.8
Held to Maturity - Amortized Cost
Due in One Year or Less	4,325.8	6,638.2
Due After One Year Through Five Years	6,859.0	7,066.0
Due After Five Years Through Ten Years	199.7	567.9
Due After Ten Years	103.3	81.9
Total	11,487.8	14,354.0
Available for Sale - Value
Due in One Year or Less	9,161.7	9,162.8
Due After One Year Through Five Years	19,284.6	20,943.9
Due After Five Years Through Ten Years	8,480.7	5,740.8
Due After Ten Years	1,993.2	1,041.3
Total	38,920.2	36,888.8
Held to Maturity - Fair Value
Due in One Year or Less	4,326.9	6,635.5
Due After One Year Through Five Years	6,905.7	7,040.0
Due After Five Years Through Ten Years	190.9	553.0
Due After Ten Years	47.9	38.5
Total	$ 11,471.4	$ 14,267.0