Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		12 Months Ended
	Jan. 01, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent		$ 4,959,000,000	$ 4,883,000,000	$ 4,608,000,000	$ 4,409,000,000
Initial ACL on PCD Loans	$ 50,300,000	$ 50,300,000
Other than temporary impairment losses			0	0
Allowances for impaired commercial loans			1,000,000.0
Core deposit intangibles amortization period, years		10 years
Largest amount recognized in the financial statement of tax benefit threshold, minimum		greater than 50%
Marketing expense		$ 12,600,000	13,100,000	12,800,000
Vesting period of awards issued, years		3 years
Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent		$ 869,000,000	798,000,000	$ 576,000,000	$ 368,000,000
Accounting Standards Update 2016-13 | Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent	(51,000,000)
Impact of ASC 326 Adoption
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent			(50,000,000)
Impact of ASC 326 Adoption | Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent			$ (50,000,000)
Impact of ASC 326 Adoption | Accounting Standards Update 2016-13 | Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in stockholders' equity attributable to parent	$ (50,600,000)
Restricted Stock Units (RSUs)
Summary Of Significant Accounting Policies [Line Items]
Vesting period of awards issued, years		3 years
Commercial loan
Summary Of Significant Accounting Policies [Line Items]
Threshold period past due for non-accrual loans		90 days
Installment Loans
Summary Of Significant Accounting Policies [Line Items]
Threshold period past due for non-accrual loans		120 days
Residential Mortgages and Consumer Lines of Credit
Summary Of Significant Accounting Policies [Line Items]
Threshold period past due for non-accrual loans		180 days
Minimum
Summary Of Significant Accounting Policies [Line Items]
Useful life of the asset, years		3 years
Estimated useful life of intangibles, years		8 years
Minimum | Commercial loan
Summary Of Significant Accounting Policies [Line Items]
Financing receivable, individual reserves in default, threshold		$ 1,000,000.0
Maximum
Summary Of Significant Accounting Policies [Line Items]
Useful life of the asset, years		39 years
Estimated useful life of intangibles, years		13 years
Maximum | Commercial and Consumer Loans
Summary Of Significant Accounting Policies [Line Items]
Financing receivable, individual reserves in default, threshold		$ 1,000,000.0