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, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Balance at December 31	$ 686.4	$ 1,596.5
Average Balance During the Year	$ 1,067.4	$ 1,253.1
Average Interest Rate Earned During the Year	0.33%	0.31%
Maximum Month-End Balance During the Year	$ 1,819.1	$ 2,055.6