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, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Balance at December 31	$ 489.7	$ 168.3
Average Balance During the Year	$ 339.0	$ 525.2
Average Interest Rate Paid During the Year	1.89%	1.48%
Maximum Month-End Balance During the Year	$ 489.7	$ 981.3