Loans - Summary of Credit Concentration by Region US States (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Product Information [Line Items]
Loans	$ 115,557	$ 110,345
Percentage concentration of gross loans by U.S. State	100.00%	100.00%
Installment [Member]
Product Information [Line Items]
Loans	$ 4,112	$ 4,747
Percentage concentration of gross loans by U.S. State	100.00%	100.00%
Installment [Member] | California [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	12.00%	12.00%
Installment [Member] | Texas [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	9.00%	9.00%
Installment [Member] | New York [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	7.00%	7.00%
Installment [Member] | Florida [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	7.00%	7.00%
Installment [Member] | Illinois [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	4.00%	4.00%
Installment [Member] | Other [member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	61.00%	61.00%
Credit card [Member]
Product Information [Line Items]
Loans	$ 2,908	$ 1,858
Percentage concentration of gross loans by U.S. State	100.00%	100.00%
Credit card [Member] | California [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	19.00%	21.00%
Credit card [Member] | Texas [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	9.00%	9.00%
Credit card [Member] | New York [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	8.00%	8.00%
Credit card [Member] | Florida [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	8.00%	8.00%
Credit card [Member] | Illinois [Member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	4.00%	4.00%
Credit card [Member] | Other [member]
Product Information [Line Items]
Percentage concentration of gross loans by U.S. State	52.00%	50.00%