Premises and Equipment, Net and Other Assets - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Percentage of equity investment	50.00%
Low income housing tax credit investments	$ 16.9	$ 15.2
Tax credits and other tax benefits recognized from LIHTC investments	4.4	4.2	$ 4.0
Amortization expense resulting from LIHTC investments	3.5	$ 2.9	$ 2.6
Unfunded commitment on investments	$ 15.3