Premises and Equipment, Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	$ 621,976	$ 601,957
Accumulated depreciation and amortization	(334,271)	(310,777)
Total premises and equipment, net	287,705	291,180
Depreciation and amortization	24,800	24,400	$ 21,100
Land
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	77,235	78,116
Buildings
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	210,335	210,012
Leasehold improvements
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	79,217	73,405
Furniture and equipment
Property, Plant and Equipment [Line Items]
Premises and equipment, gross	$ 255,189	$ 240,424