CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Operating activities:
Net income	$ 556	$ 150	$ 1,278	$ 495
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization			63	55
Provision for credit losses	26	23	78	57
Share-based compensation			229	227
Other			13	0
Changes in operating assets and liabilities:
Securities segregated under federal and other regulations			(1,193)	0
Receivables from brokers, dealers, and clearing organizations			19	(50)
Receivables from users, net			(5,814)	(1,971)
Securities borrowed			(3,371)	(2,102)
Deposits with clearing organizations			(951)	(126)
Current and non-current prepaid expenses			(42)	(41)
Current and non-current deferred customer match incentives			(226)	(202)
Other current and non-current assets			437	(11)
Accounts payable and accrued expenses			(75)	28
Payables to users			3,754	1,167
Securities loaned			8,203	3,759
Other current and non-current liabilities			173	(42)
Net cash provided by operating activities			2,575	1,243
Investing activities:
Purchases of property, software, and equipment			(13)	(9)
Capitalization of internally developed software			(28)	(26)
Consideration transferred for business acquisitions			(399)	(6)
Cash, cash equivalents, and segregated cash acquired in business acquisitions			1,193	0
Purchases of held-to-maturity investments			0	(469)
Proceeds from maturities of held-to-maturity investments			347	439
Purchases of credit card receivables by Credit Card Funding Trust			(2,917)	(239)
Collections of purchased credit card receivables			2,526	130
Asset acquisition, net of cash acquired			0	(3)
Other			(10)	1
Net cash provided by (used in) investing activities			699	(182)
Financing activities:
Proceeds from exercise of stock options			14	10
Proceeds from issuance of common stock under the Employee Share Purchase Plan			15	10
Taxes paid related to net share settlement of equity awards			(413)	(155)
Repurchase of Class A common stock			(553)	(97)
Draws on credit facilities			2,701	12
Repayments on credit facilities			(2,701)	(12)
Borrowings by the Credit Card Funding Trust			245	95
Change in principal collected from customers due to Coastal Bank			0	(15)
Repayments on borrowings by the Credit Card Funding Trust			0	(1)
Payments of debt issuance costs			(16)	(14)
Net cash used in financing activities			(708)	(167)
Effect of foreign exchange rate changes on cash and cash equivalents			8	1
Net increase in cash, cash equivalents, segregated cash, and restricted cash			2,574	895
Cash, cash equivalents, segregated cash, and restricted cash, beginning of the period			8,695	9,346	$ 9,346
Cash, cash equivalents, segregated cash, and restricted cash, end of the period	11,269	10,241	11,269	10,241	8,695
Reconciliation of cash, cash equivalents, segregated cash and restricted cash, end of the period:
Cash and cash equivalents, end of the period	4,331	4,611	4,331	4,611	4,332
Segregated cash and cash equivalents, end of the period	6,853	5,547	6,853	5,547
Restricted cash in other current assets, end of the period	68	67	68	67
Restricted cash in other non-current assets, end of the period	17	16	17	16
Cash, cash equivalents, segregated cash and restricted cash, end of the period	$ 11,269	$ 10,241	11,269	10,241	$ 8,695
Supplemental disclosures:
Cash paid for interest			20	12
Cash paid for income taxes, net of refund received			$ 83	$ 14