Loans - Consumer Banking: Net Charge-Offs and Nonperforming Loans (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Nonperforming loans	$ 780		$ 780		$ 813
Nonperforming loans rate	0.30%		0.30%		0.30%
Consumer Banking
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 172	$ 198	$ 393	$ 421
Net charge-offs rate	1.15%	1.19%	1.32%	1.19%
Nonperforming loans	$ 397		$ 397		$ 479
Nonperforming loans rate	0.66%		0.66%		0.81%
Consumer Banking | Auto
Loans and Leases Receivable Disclosure [Line Items]
Net charge-offs	$ 155	$ 182	$ 358	$ 390
Net charge-offs rate	1.09%	1.32%	1.26%	1.42%
Nonperforming loans	$ 369		$ 369		$ 449
Nonperforming loans rate	0.64%		0.64%		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	$ 16	$ 35	$ 32
Net charge-offs rate	2.42%	2.07%	2.49%	1.97%
Nonperforming loans	$ 28		$ 28		$ 30
Nonperforming loans rate	1.02%		1.02%		1.04%