Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Available for Sale - Cost
Due in One Year or Less	$ 9,324.4	$ 9,206.2
Due After One Year Through Five Years	19,674.2	21,012.7
Due After Five Years Through Ten Years	7,188.4	5,774.1
Due After Ten Years	1,499.1	1,048.8
Total	37,686.1	37,041.8
Held to Maturity - Amortized Cost
Due in One Year or Less	3,374.3	6,638.2
Due After One Year Through Five Years	6,803.4	7,066.0
Due After Five Years Through Ten Years	76.4	567.9
Due After Ten Years	97.1	81.9
Total	10,351.2	14,354.0
Available for Sale - Value
Due in One Year or Less	9,325.6	9,162.8
Due After One Year Through Five Years	19,799.6	20,943.9
Due After Five Years Through Ten Years	7,234.5	5,740.8
Due After Ten Years	1,507.8	1,041.3
Total	37,867.5	36,888.8
Held to Maturity - Fair Value
Due in One Year or Less	3,374.9	6,635.5
Due After One Year Through Five Years	6,837.9	7,040.0
Due After Five Years Through Ten Years	66.3	553.0
Due After Ten Years	46.3	38.5
Total	$ 10,325.4	$ 14,267.0