Reserves for Credit Losses - Changes in Loan Reserve (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2025	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provisions for credit losses		$ 1,287	$ 1,356	$ 3,842	$ 3,893
Card Member Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		5,767	5,321	5,679	5,118	$ 5,118
Provisions for credit losses	$ (144)	1,030	1,114	3,025	3,098
Other		(1)	6	25	(7)
Ending Balance	$ 5,767	5,868	5,588	5,868	5,588	$ 5,679
Card Member Loans | Principal
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Net write-offs		(760)	(701)	(2,349)	(2,159)
Recoveries		261	192	724	530
Card Member Loans | Interest and fees
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Net write-offs		$ (168)	$ (152)	$ (512)	$ (462)