Loans and Leases - Summary of Commercial Loans and Leases by Credit Quality (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	$ 25,459	$ 23,289
Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	17,470	14,721
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	9,731	8,960
Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	7,214	5,308
Commercial leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	485	432
Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	40	21
Originated Loans And Leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		20,397
Originated Loans And Leases | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		12,630
Originated Loans And Leases | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		7,114
Originated Loans And Leases | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		5,063
Originated Loans And Leases | Commercial leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		432
Originated Loans And Leases | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		21
Acquired Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		2,892
Acquired Loans | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		2,091
Acquired Loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		1,846
Acquired Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		245
Pass | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	6,713
Pass | Commercial leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	459
Pass | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	39
Pass | Originated Loans And Leases | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		12,032
Pass | Originated Loans And Leases | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		6,821
Pass | Originated Loans And Leases | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		4,768
Pass | Originated Loans And Leases | Commercial leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		423
Pass | Originated Loans And Leases | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		20
Pass | Acquired Loans | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		1,804
Pass | Acquired Loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		1,603
Pass | Acquired Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		201
Special Mention | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	218
Special Mention | Commercial leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	13
Special Mention | Originated Loans And Leases | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		323
Special Mention | Originated Loans And Leases | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		171
Special Mention | Originated Loans And Leases | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		149
Special Mention | Originated Loans And Leases | Commercial leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		3
Special Mention | Originated Loans And Leases | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		0
Special Mention | Acquired Loans | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		135
Special Mention | Acquired Loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		116
Special Mention | Acquired Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		19
Substandard | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	283
Substandard | Commercial leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	13
Substandard | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases	$ 1
Substandard | Originated Loans And Leases | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		272
Substandard | Originated Loans And Leases | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		121
Substandard | Originated Loans And Leases | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		144
Substandard | Originated Loans And Leases | Commercial leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		6
Substandard | Originated Loans And Leases | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		1
Substandard | Acquired Loans | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		152
Substandard | Acquired Loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		127
Substandard | Acquired Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		25
Doubtful | Originated Loans And Leases | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		3
Doubtful | Originated Loans And Leases | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		1
Doubtful | Originated Loans And Leases | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		2
Doubtful | Originated Loans And Leases | Commercial leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		0
Doubtful | Originated Loans And Leases | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		0
Doubtful | Acquired Loans | Total commercial loans and leases
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		0
Doubtful | Acquired Loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		0
Doubtful | Acquired Loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans and Leases		$ 0