Remaining Maturity of Securities Available for Sale (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Available for Sale - Amortized Cost
Due in One Year or Less	$ 6,783.0
Due After One Year Through Five Years	19,052.4
Due After Five Years Through Ten Years	1,357.3
Due After Ten Years	704.1
Total	27,896.8
Held to Maturity - Amortized Cost
Due in One Year or Less	670.3
Due After One Year Through Five Years	284.1
Due After Five Years Through Ten Years	55.2
Due After Ten Years	38.4
Total	1,048.0	799.2
Available for Sale - Fair Value
Due in One Year or Less	6,802.5
Due After One Year Through Five Years	19,240.6
Due After Five Years Through Ten Years	1,375.2
Due After Ten Years	717.8
Total	28,136.1
Held to Maturity - Fair Value
Due in One Year or Less	673.1
Due After One Year Through Five Years	297.4
Due After Five Years Through Ten Years	60.7
Due After Ten Years	33.0
Fair Value	$ 1,064.2	$ 817.1