Fair Value Measurements - Fair Values of Corporation's Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial Assets
Cash and cash equivalents	$ 488	$ 479	$ 371	$ 489
Debt securities available for sale	3,341
Securities available for sale	3,341	2,765
Securities held to maturity	3,254	3,242
Securities held to maturity	3,155	3,218
Derivative financial instruments	43	31
Financial Liabilities
Deposits	23,455	22,400
Short-term borrowings	4,129	3,679
Long-term borrowings	627	668
Derivative liabilities	39	29
Carrying Amount
Financial Assets
Cash and cash equivalents	488	479
Debt securities available for sale	3,341
Securities available for sale		2,765
Securities held to maturity	3,254	3,242
Net loans and leases, including loans held for sale	21,995	20,917
Loan servicing rights	41	34
Derivative financial instruments	43	31
Accrued interest receivable	101	94
Financial Liabilities
Deposits	23,455	22,400
Short-term borrowings	4,129	3,679
Long-term borrowings	627	668
Derivative liabilities	39	29
Accrued interest payable	20	12
Fair Value
Financial Assets
Cash and cash equivalents	488	479
Debt securities available for sale	3,341
Securities available for sale		2,765
Securities held to maturity	3,155	3,218
Net loans and leases, including loans held for sale	21,742	20,661
Loan servicing rights	45	38
Derivative financial instruments	43	31
Accrued interest receivable	101	94
Financial Liabilities
Deposits	23,411	22,359
Short-term borrowings	4,130	3,679
Long-term borrowings	618	675
Derivative liabilities	39	29
Accrued interest payable	20	12
Level 1
Financial Assets
Cash and cash equivalents	488	479
Debt securities available for sale	0
Securities available for sale		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	101	94
Financial Liabilities
Deposits	18,142	17,779
Short-term borrowings	4,130	3,679
Long-term borrowings	0	0
Derivative liabilities	0	0
Accrued interest payable	20	12
Level 2
Financial Assets
Cash and cash equivalents	0	0
Debt securities available for sale	3,341
Securities available for sale		2,765
Securities held to maturity	3,155	3,218
Net loans and leases, including loans held for sale	14	56
Loan servicing rights	0	0
Derivative financial instruments	42	29
Accrued interest receivable	0	0
Financial Liabilities
Deposits	5,269	4,580
Short-term borrowings	0	0
Long-term borrowings	0	0
Derivative liabilities	39	29
Accrued interest payable	0	0
Level 3
Financial Assets
Cash and cash equivalents	0	0
Debt securities available for sale	0
Securities available for sale		0
Securities held to maturity	0	0
Net loans and leases, including loans held for sale	21,728	20,605
Loan servicing rights	45	38
Derivative financial instruments	1	2
Accrued interest receivable	0	0
Financial Liabilities
Deposits	0	0
Short-term borrowings	0	0
Long-term borrowings	618	675
Derivative liabilities	0	0
Accrued interest payable	$ 0	$ 0