Securities Purchased Under Agreements to Resell and Securities Sold Under Agreements to Repurchase - Securities Sold under Agreements to Repurchase (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Balance at December 31	$ 39.8	$ 489.7
Average Balance During the Year	$ 218.3	$ 339.0
Average Interest Rate Paid During the Year	0.47%	1.89%
Maximum Month-End Balance During the Year	$ 269.8	$ 489.7