Fair Value Measurements - Fair Values of Corporation's Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financial Assets
Cash and cash equivalents	$ 599	$ 488
Debt securities available for sale	3,289	3,341
Securities held to maturity	3,275	3,254
Securities held to maturity	3,305	3,155
Derivative financial instruments	154	43
Financial Liabilities
Deposits	24,786	23,455
Short-term borrowings	3,216	4,129
Long-term borrowings	1,340	627
Long-term borrowings	249
Derivative liabilities	25	39
Carrying Amount
Financial Assets
Cash and cash equivalents	599	488
Debt securities available for sale	3,289	3,341
Securities held to maturity	3,275	3,254
Net loans and leases, including loans held for sale	23,144	21,995
Loan servicing rights	46	41
Derivative financial instruments	154	43
Accrued interest receivable	109	101
Financial Liabilities
Deposits	24,786	23,455
Short-term borrowings	3,216	4,129
Long-term borrowings	1,340	627
Derivative liabilities	25	39
Accrued interest payable	21	20
Fair Value
Financial Assets
Cash and cash equivalents	599	488
Debt securities available for sale	3,289	3,341
Securities held to maturity	3,305	3,155
Net loans and leases, including loans held for sale	22,930	21,742
Loan servicing rights	48	45
Derivative financial instruments	154	43
Accrued interest receivable	109	101
Financial Liabilities
Deposits	24,797	23,411
Short-term borrowings	3,219	4,130
Long-term borrowings	1,355	618
Derivative liabilities	25	39
Accrued interest payable	21	20
Level 1
Financial Assets
Cash and cash equivalents	599	488
Debt securities available for sale	0	0
Securities held to maturity	0	0
Net loans and leases, including loans held for sale	0	0
Loan servicing rights	0	0
Derivative financial instruments	0	0
Accrued interest receivable	109	101
Financial Liabilities
Deposits	20,058	18,142
Short-term borrowings	3,219	4,130
Long-term borrowings	0	0
Derivative liabilities	0	0
Accrued interest payable	21	20
Level 2
Financial Assets
Cash and cash equivalents	0	0
Debt securities available for sale	3,289	3,341
Securities held to maturity	3,305	3,155
Net loans and leases, including loans held for sale	41	14
Loan servicing rights	0	0
Derivative financial instruments	151	42
Accrued interest receivable	0	0
Financial Liabilities
Deposits	4,739	5,269
Short-term borrowings	0	0
Long-term borrowings	0	0
Derivative liabilities	25	39
Accrued interest payable	0	0
Level 3
Financial Assets
Cash and cash equivalents	0	0
Debt securities available for sale	0	0
Securities held to maturity	0	0
Net loans and leases, including loans held for sale	22,889	21,728
Loan servicing rights	48	45
Derivative financial instruments	3	1
Accrued interest receivable	0	0
Financial Liabilities
Deposits	0	0
Short-term borrowings	0	0
Long-term borrowings	1,355	618
Derivative liabilities	0	0
Accrued interest payable	$ 0	$ 0