Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available for Sale - Cost
Due in One Year or Less	$ 6,343.8
Due After One Year Through Five Years	20,521.1
Due After Five Years Through Ten Years	6,105.8
Due After Ten Years	898.3
Total	33,869.0
Held to Maturity - Amortized Cost
Due in One Year or Less	3,355.6
Due After One Year Through Five Years	6,995.0
Due After Five Years Through Ten Years	399.5
Due After Ten Years	60.9
Total	10,811.0	$ 8,921.1
Available for Sale - Value
Due in One Year or Less	6,337.3
Due After One Year Through Five Years	20,506.8
Due After Five Years Through Ten Years	6,108.4
Due After Ten Years	892.4
Total	33,844.9
Held to Maturity - Fair Value
Due in One Year or Less	3,357.6
Due After One Year Through Five Years	6,991.9
Due After Five Years Through Ten Years	393.3
Due After Ten Years	35.9
Total	$ 10,778.7	$ 8,905.1