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, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Balance at December 31	$ 168.3	$ 834.0
Average Balance During the Year	$ 525.2	$ 738.9
Average Interest Rate Paid During the Year	1.48%	0.81%
Maximum Month-End Balance During the Year	$ 981.3	$ 834.0