Remaining Maturity of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 32,367.8	$ 29,514.1
Held to Maturity - Amortized Cost
Due in One Year or Less	1,908.8	1,503.5
Due After One Year Through Five Years	3,271.1	2,602.8
Due After Five Years Through Ten Years	14.9	23.5
Due After Ten Years	53.5	41.0
Amortized Cost	5,248.3	4,170.8
Held to Maturity - Fair Value
Due in One Year or Less	1,913.2	1,504.7
Due After One Year Through Five Years	3,278.3	2,622.3
Due After Five Years Through Ten Years	11.7	21.5
Due After Ten Years	24.3	27.6
Total	$ 5,227.5	$ 4,176.1