Securities Purchased Under Agreements to Resell and Securities Sold Under Agreements to Repurchase - Securities Sold under Agreements to Repurchase (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Balance at December 31	$ 834.0	$ 473.7
Average Balance During the Year	$ 738.9	$ 847.1
Average Interest Rate Paid During the Year	0.81%	0.27%
Maximum Month-End Balance During the Year	$ 834.0	$ 565.5