Premises and Equipment - Major Categories of Banking Premises and Equipment and Accumulated Depreciation (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2020
Buildings [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	15 years
Buildings [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	39 years
Furnishings [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Furnishings [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	15 years