LOANS AND LEASES - Commercial Loans and Leases by Credit Quality (Detail) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount	$ 26,162	$ 23,289
Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount	9,305	8,960
Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount	7,709	5,308
Commercial Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount	497	432
Other [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount	40	21
Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount	$ 17,551	14,721
Originated Loans And Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		20,397
Originated Loans And Leases [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		7,114
Originated Loans And Leases [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		5,063
Originated Loans And Leases [Member] | Commercial Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		432
Originated Loans And Leases [Member] | Other [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		21
Originated Loans And Leases [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		12,630
Acquired Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		2,892
Acquired Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		1,846
Acquired Loans [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		245
Acquired Loans [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		2,091
Pass [Member] | Originated Loans And Leases [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		6,821
Pass [Member] | Originated Loans And Leases [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		4,768
Pass [Member] | Originated Loans And Leases [Member] | Commercial Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		423
Pass [Member] | Originated Loans And Leases [Member] | Other [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		20
Pass [Member] | Originated Loans And Leases [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		12,032
Pass [Member] | Acquired Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		1,603
Pass [Member] | Acquired Loans [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		201
Pass [Member] | Acquired Loans [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		1,804
Special Mention [Member] | Originated Loans And Leases [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		171
Special Mention [Member] | Originated Loans And Leases [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		149
Special Mention [Member] | Originated Loans And Leases [Member] | Commercial Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		3
Special Mention [Member] | Originated Loans And Leases [Member] | Other [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		0
Special Mention [Member] | Originated Loans And Leases [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		323
Special Mention [Member] | Acquired Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		116
Special Mention [Member] | Acquired Loans [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		19
Special Mention [Member] | Acquired Loans [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		135
Substandard [Member] | Originated Loans And Leases [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		121
Substandard [Member] | Originated Loans And Leases [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		144
Substandard [Member] | Originated Loans And Leases [Member] | Commercial Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		6
Substandard [Member] | Originated Loans And Leases [Member] | Other [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		1
Substandard [Member] | Originated Loans And Leases [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		272
Substandard [Member] | Acquired Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		127
Substandard [Member] | Acquired Loans [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		25
Substandard [Member] | Acquired Loans [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		152
Doubtful [Member] | Originated Loans And Leases [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		1
Doubtful [Member] | Originated Loans And Leases [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		2
Doubtful [Member] | Originated Loans And Leases [Member] | Commercial Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		0
Doubtful [Member] | Originated Loans And Leases [Member] | Other [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		0
Doubtful [Member] | Originated Loans And Leases [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		3
Doubtful [Member] | Acquired Loans [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		0
Doubtful [Member] | Acquired Loans [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		0
Doubtful [Member] | Acquired Loans [Member] | Total Commercial Loans and Leases [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amortized cost loans and leases, carrying amount		$ 0