Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Available for Sale - Cost
Due in One Year or Less	$ 6,351.0
Due After One Year Through Five Years	20,797.9
Due After Five Years Through Ten Years	6,014.9
Due After Ten Years	1,022.5
Total	34,186.3
Held to Maturity - Amortized Cost
Due in One Year or Less	2,854.3
Due After One Year Through Five Years	6,127.7
Due After Five Years Through Ten Years	205.0
Due After Ten Years	60.9
Total	9,247.9	$ 8,921.1
Available for Sale - Value
Due in One Year or Less	6,345.0
Due After One Year Through Five Years	20,789.5
Due After Five Years Through Ten Years	6,017.5
Due After Ten Years	1,015.8
Total	34,167.8
Held to Maturity - Fair Value
Due in One Year or Less	2,855.4
Due After One Year Through Five Years	6,131.9
Due After Five Years Through Ten Years	203.9
Due After Ten Years	34.4
Total	$ 9,225.6	$ 8,905.1