Securities - Remaining Maturity of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Held to Maturity - Amortized Cost
Due in One Year or Less	$ 5,691.9	$ 3,631.6
Due After One Year Through Five Years	6,667.8	5,072.7
Due After Five Years Through Ten Years	612.2	158.7
Due After Ten Years	77.1	58.1
AMORTIZED COST	13,049.0	8,921.1
Held to Maturity - Fair Value
Due in One Year or Less	5,695.8	3,635.9
Due After One Year Through Five Years	6,663.9	5,081.6
Due After Five Years Through Ten Years	606.3	156.1
Due After Ten Years	44.9	31.5
Total	$ 13,010.9	$ 8,905.1