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, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Balance at December 31	$ 1,031.2	$ 1,303.3
Average Balance During the Year	$ 1,478.3	$ 1,832.0
Average Interest Rate Earned During the Year	2.22%	1.48%
Maximum Month-End Balance During the Year	$ 1,942.0	$ 2,064.1