Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Goodwill from business combinations		$ 4,298,000
Goodwill impairment	$ 0	0	$ 0
Impairment of core deposits and intangibles	0	0	0
Amortization expense - other intangible assets	$ 10,000,000	11,300,000	9,200,000
Core deposits
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years	11 years
Other
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years	20 years
Residential mortgage
Finite-Lived Intangible Assets [Line Items]
Aggregate principal balances of mortgage loans serviced	$ 2,800,000,000	2,500,000,000	$ 2,100,000,000
Wealth Management
Finite-Lived Intangible Assets [Line Items]
Goodwill from business combinations		701,000
Segments excluding Wealth Management
Finite-Lived Intangible Assets [Line Items]
Goodwill from business combinations		$ 3,600,000