Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available for Sale - Cost
Due in One Year or Less	$ 8,595.3
Due After One Year Through Five Years	20,929.2
Due After Five Years Through Ten Years	5,626.7
Due After Ten Years	1,245.8
Total	36,397.0	$ 33,864.8
Held to Maturity - Amortized Cost
Due in One Year or Less	6,324.1
Due After One Year Through Five Years	7,519.4
Due After Five Years Through Ten Years	520.2
Due After Ten Years	81.7
Total	14,445.4	13,049.0
Available for Sale - Value
Due in One Year or Less	8,536.1
Due After One Year Through Five Years	20,782.0
Due After Five Years Through Ten Years	5,581.0
Due After Ten Years	1,232.0
Total	36,131.1	33,742.1
Held to Maturity - Fair Value
Due in One Year or Less	6,324.2
Due After One Year Through Five Years	7,476.9
Due After Five Years Through Ten Years	509.9
Due After Ten Years	44.1
Total	$ 14,355.1	$ 13,010.9