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, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Balance at December 31	$ 1,596.5	$ 707.8
Average Balance During the Year	$ 1,253.1	$ 835.0
Average Interest Rate Earned During the Year	0.31%	2.10%
Maximum Month-End Balance During the Year	$ 2,055.6	$ 1,290.0