Loans and Leases - Summary of Consumer Loans by Payment Status (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Credit Quality [Line Items]
Total Loans and Leases	$ 23,289	$ 22,153
Direct installment
Credit Quality [Line Items]
Total Loans and Leases	1,821	1,764
Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	3,374	3,113
Indirect installment
Credit Quality [Line Items]
Total Loans and Leases	1,922	1,933
Originated Loans and Leases
Credit Quality [Line Items]
Total Loans and Leases	20,397	18,062
Originated Loans and Leases | Direct installment
Credit Quality [Line Items]
Total Loans and Leases	1,758	1,668
Originated Loans and Leases | Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	2,995	2,612
Originated Loans and Leases | Indirect installment
Credit Quality [Line Items]
Total Loans and Leases	1,922	1,933
Loans Acquired in a Business Combination
Credit Quality [Line Items]
Total Loans and Leases	2,892	4,091
Loans Acquired in a Business Combination | Direct installment
Credit Quality [Line Items]
Total Loans and Leases	63	96
Loans Acquired in a Business Combination | Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	379	501
Total consumer loans | Originated Loans and Leases
Credit Quality [Line Items]
Total Loans and Leases	7,767	7,332
Total consumer loans | Originated Loans and Leases | Performing
Credit Quality [Line Items]
Total Loans and Leases	7,728	7,297
Total consumer loans | Originated Loans and Leases | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	39	35
Total consumer loans | Originated Loans and Leases | Direct installment
Credit Quality [Line Items]
Total Loans and Leases	1,758	1,668
Total consumer loans | Originated Loans and Leases | Direct installment | Performing
Credit Quality [Line Items]
Total Loans and Leases	1,745	1,654
Total consumer loans | Originated Loans and Leases | Direct installment | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	13	14
Total consumer loans | Originated Loans and Leases | Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	2,995	2,612
Total consumer loans | Originated Loans and Leases | Residential mortgages | Performing
Credit Quality [Line Items]
Total Loans and Leases	2,978	2,598
Total consumer loans | Originated Loans and Leases | Residential mortgages | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	17	14
Total consumer loans | Originated Loans and Leases | Indirect installment
Credit Quality [Line Items]
Total Loans and Leases	1,922	1,933
Total consumer loans | Originated Loans and Leases | Indirect installment | Performing
Credit Quality [Line Items]
Total Loans and Leases	1,919	1,931
Total consumer loans | Originated Loans and Leases | Indirect installment | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	3	2
Total consumer loans | Originated Loans and Leases | Consumer lines of credit
Credit Quality [Line Items]
Total Loans and Leases	1,092	1,119
Total consumer loans | Originated Loans and Leases | Consumer lines of credit | Performing
Credit Quality [Line Items]
Total Loans and Leases	1,086	1,114
Total consumer loans | Originated Loans and Leases | Consumer lines of credit | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	6	5
Total consumer loans | Loans Acquired in a Business Combination
Credit Quality [Line Items]
Total Loans and Leases	801	1,060
Total consumer loans | Loans Acquired in a Business Combination | Performing
Credit Quality [Line Items]
Total Loans and Leases	800	1,059
Total consumer loans | Loans Acquired in a Business Combination | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	1	1
Total consumer loans | Loans Acquired in a Business Combination | Direct installment
Credit Quality [Line Items]
Total Loans and Leases	63	96
Total consumer loans | Loans Acquired in a Business Combination | Direct installment | Performing
Credit Quality [Line Items]
Total Loans and Leases	63	96
Total consumer loans | Loans Acquired in a Business Combination | Direct installment | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	0	0
Total consumer loans | Loans Acquired in a Business Combination | Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	379	501
Total consumer loans | Loans Acquired in a Business Combination | Residential mortgages | Performing
Credit Quality [Line Items]
Total Loans and Leases	379	501
Total consumer loans | Loans Acquired in a Business Combination | Residential mortgages | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	0	0
Total consumer loans | Loans Acquired in a Business Combination | Consumer lines of credit
Credit Quality [Line Items]
Total Loans and Leases	359	463
Total consumer loans | Loans Acquired in a Business Combination | Consumer lines of credit | Performing
Credit Quality [Line Items]
Total Loans and Leases	358	462
Total consumer loans | Loans Acquired in a Business Combination | Consumer lines of credit | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	$ 1	$ 1