Borrowings - Interest Expense on Borrowings (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
FHLB advances	$ 50.1	$ 54.5	$ 31.5
Federal funds purchased	24.6	13.6	7.1
Customer repurchase agreements	2.2	1.0	0.6
Other borrowings	0.1	1.8	2.1
Total	$ 77.0	$ 70.9	$ 41.3