Basis of Presentation and Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Basis Of Presentation And Significant Accounting Policies [Line Items]
Loan placed on nonaccrual when past due, number of days	90 days
Nonaccrual period for loans	90 days
Percentage of income tax examination likelihood of tax benefits	50.00%
Lease liability	$ 99,500,000
Lease, right-of-use asset	95,477,000
Cumulative-effect adjustment, increase in retained earnings		$ 6,322,000	$ (4,179,000)
Derivative notional amount	665,500,000	1,482,000,000
Estimated increase in allowance for credit losses	2,000,000.0	$ 0	$ 2,700,000
Accounting Standards Update 2016-02 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Lease liability				$ 122,900,000
Lease, right-of-use asset				118,700,000
Cumulative-effect adjustment, increase in retained earnings				$ 6,300,000
Accounting Standards Update 2018-16 [Member] | LIBOR [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Derivative notional amount	4,800,000,000
Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Value of small commercial loans on nonaccrual status or 90 days or more delinquent	$ 250,000
Maturity of short-term securities sold under agreements to repurchase	4 days
Maximum [Member] | Accounting Standards Update 2016-13 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated increase in allowance for credit losses	$ 45,000,000
Expected increase in off balance sheet exposures	$ 8,000,000
Maximum [Member] | Core Deposits and Other Intangible Assets [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful lives of core deposits and customer relationships	15 years
Maximum [Member] | Building and Building Improvements [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years	39 years
Maximum [Member] | Furniture and Equipment [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years	7 years
Minimum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Maturity of short-term securities sold under agreements to repurchase	1 day
Minimum [Member] | Accounting Standards Update 2016-13 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated increase in allowance for credit losses	$ 35,000,000
Expected increase in off balance sheet exposures	$ 3,000,000
Minimum [Member] | Core Deposits and Other Intangible Assets [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful lives of core deposits and customer relationships	5 years
Minimum [Member] | Building and Building Improvements [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years	15 years
Minimum [Member] | Furniture and Equipment [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years	3 years