Loans - Consumer Banking: Net Charge-Offs and Nonperforming Loans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Nonperforming loans	$ 929		$ 929		$ 813
Nonperforming loans rate	0.40%		0.40%		0.30%
Consumer Banking
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 251	$ 262	$ 644	$ 683
Net charge-offs rate	1.64%	1.77%	1.43%	1.36%
Nonperforming loans	$ 457		$ 457		$ 479
Nonperforming loans rate	0.74%		0.74%		0.81%
Consumer Banking | Auto
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 234	$ 243	$ 592	$ 633
Net charge-offs rate	1.60%	1.73%	1.38%	1.53%
Nonperforming loans	$ 432		$ 432		$ 449
Nonperforming loans rate	0.73%		0.73%		0.80%
Consumer Banking | Home loan
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 0	$ 0	$ 0	$ (1)
Net charge-offs rate	0.00%	0.00%	0.00%	(0.02%)
Consumer Banking | Retail banking
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 17	$ 19	$ 52	$ 51
Net charge-offs rate	2.55%	2.62%	2.51%	2.18%
Nonperforming loans	$ 25		$ 25		$ 30
Nonperforming loans rate	0.91%		0.91%		1.04%