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, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Balance at December 31	$ 707.8	$ 1,031.2
Average Balance During the Year	$ 835.0	$ 1,478.3
Average Interest Rate Earned During the Year	2.10%	2.22%
Maximum Month-End Balance During the Year	$ 1,290.0	$ 1,942.0