Fair Value Measurements - Fair Values of Corporation's Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financial Assets
Cash and cash equivalents	$ 1,383	$ 599
Debt securities available for sale	3,463	3,289
Securities held to maturity	2,868	3,275
Securities held to maturity	2,973	3,305
Net Loans and Leases	25,096	23,093
Derivative financial instruments	376	154
Financial Liabilities
Deposits	29,122	24,786
Short-term borrowings	1,804	3,216
Long-term borrowings	1,095	1,340
Long-term borrowings	548
Derivative liabilities	40	25
Carrying Amount
Financial Assets
Cash and cash equivalents	1,383	599
Debt securities available for sale	3,463	3,289
Securities held to maturity	2,868	3,275
Net Loans and Leases	25,250	23,144
Loan servicing rights	39	46
Derivative financial instruments	376	154
Accrued interest receivable	90	109
Financial Liabilities
Deposits	29,122	24,786
Short-term borrowings	1,804	3,216
Long-term borrowings	1,095	1,340
Derivative liabilities	40	25
Accrued interest payable	13	21
Fair Value
Financial Assets
Cash and cash equivalents	1,383	599
Debt securities available for sale	3,463	3,289
Securities held to maturity	2,973	3,305
Net loans and leases, including loans held for sale	25,012	22,930
Loan servicing rights	39	48
Derivative financial instruments	376	154
Accrued interest receivable	90	109
Financial Liabilities
Deposits	29,158	24,797
Short-term borrowings	1,809	3,219
Long-term borrowings	1,068	1,355
Derivative liabilities	40	25
Accrued interest payable	13	21
Level 1
Financial Assets
Cash and cash equivalents	1,383	599
Debt securities available for sale	600	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	90	109
Financial Liabilities
Deposits	25,460	20,058
Short-term borrowings	1,809	3,219
Long-term borrowings	0	0
Derivative liabilities	0	0
Accrued interest payable	13	21
Level 2
Financial Assets
Cash and cash equivalents	0	0
Debt securities available for sale	2,863	3,289
Securities held to maturity	2,973	3,305
Net loans and leases, including loans held for sale	144	41
Loan servicing rights	0	0
Derivative financial instruments	352	151
Accrued interest receivable	0	0
Financial Liabilities
Deposits	3,698	4,739
Short-term borrowings	0	0
Long-term borrowings	0	0
Derivative liabilities	40	25
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	24,868	22,889
Loan servicing rights	39	48
Derivative financial instruments	24	3
Accrued interest receivable	0	0
Financial Liabilities
Deposits	0	0
Short-term borrowings	0	0
Long-term borrowings	1,068	1,355
Derivative liabilities	0	0
Accrued interest payable	$ 0	$ 0