Securities Purchased Under Agreements to Resell and Securities Sold Under Agreements to Repurchase - Securities Purchased under Agreements to Resell (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Balance at December 31	$ 1,303.3	$ 1,967.5
Average Balance During the Year	$ 1,832.0	$ 1,764.1
Average Interest Rate Earned During the Year	1.48%	1.04%
Maximum Month-End Balance During the Year	$ 2,064.1	$ 2,050.9