Securities Purchased under Agreements to Resell and Securities Sold Under Agreements to Repurchase (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Securities purchased under agreements to resell	$ 1,967.5	$ 1,600.0
Average Balance During the Year	$ 1,764.1	$ 1,144.7
Average Interest Rate Earned During the Year	1.04%	0.54%
Maximum Month-End Balance During the Year	$ 2,050.9	$ 1,600.0