Securities Purchased under Agreements to Resell and Securities Sold Under Agreements to Repurchase (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Securities purchased under agreements to resell	$ 1,600.0	$ 1,000.0
Average Balance During the Year	$ 1,144.7	$ 742.1
Average Interest Rate Earned During the Year	0.54%	0.45%
Maximum Month-End Balance During the Year	$ 1,600.0	$ 1,000.0