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 sold under agreements to repurchase	$ 473.7	$ 546.6
Average Balance During the Year	$ 847.1	$ 649.5
Average Interest Rate Paid During the Year	0.27%	0.05%
Maximum Month-End Balance During the Year	$ 565.5	$ 802.4