CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Operating activities:
Net income (loss)	$ 188	$ 25	$ 345	$ (486)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization			35	35
Provision for credit losses			34	15
Share-based compensation			148	707
Other			(1)	0
Changes in operating assets and liabilities:
Securities segregated under federal and other regulations			(547)	0
Receivables from brokers, dealers, and clearing organizations			(60)	(41)
Receivables from users, net			(1,538)	(111)
Securities borrowed			(615)	(443)
Deposits with clearing organizations			(213)	(37)
Current and non-current prepaid expenses			(20)	9
Current and non-current deferred customer match incentives			(196)	(6)
Other current and non-current assets			(128)	(52)
Accounts payable and accrued expenses			(26)	51
Payables to users			692	410
Securities loaned			1,544	1,148
Other current and non-current liabilities			(23)	(1)
Net cash provided by (used in) operating activities			(569)	1,198
Investing activities:
Purchases of property, software, and equipment			(2)	0
Capitalization of internally developed software			(14)	(9)
Purchases of held-to-maturity investments			(302)	(575)
Proceeds from maturities of held-to-maturity investments			289	92
Purchases of credit card receivables			(70)	0
Collections of purchased credit card receivables			48	0
Business acquisition			(6)	0
Asset acquisition, net of cash acquired			(3)	0
Other			1	10
Net cash used in investing activities			(59)	(482)
Financing activities:
Proceeds from issuance of common stock under the Employee Stock Purchase Plan			10	9
Taxes paid related to net share settlement of equity awards			(99)	(5)
Payments of debt issuance costs			(14)	(10)
Draws on credit facilities			11	10
Repayments on credit facilities			(11)	(10)
Borrowings on Credit Card Funding Trust			17	0
Repayments on Credit Card Funding Trust			(1)	0
Change in principal collected from customers due to Coastal Bank			7	0
Proceeds from exercise of stock options, net of repurchases			8	2
Net cash used in financing activities			(72)	(4)
Net increase (decrease) in cash, cash equivalents, segregated cash, and restricted cash			(700)	712
Cash, cash equivalents, segregated cash, and restricted cash, beginning of the period			9,346	9,357	$ 9,357
Cash, cash equivalents, segregated cash, and restricted cash, end of the period	8,646	10,069	8,646	10,069	9,346
Reconciliation of cash, cash equivalents, segregated cash and restricted cash, end of the period:
Cash and cash equivalents, end of the period	4,524	5,829	4,524	5,829	4,835
Segregated cash and cash equivalents, end of the period	4,037	4,220	4,037	4,220
Restricted cash in other current assets, end of the period	69	0	69	0
Restricted cash in other non-current assets, end of the period	16	20	16	20
Cash, cash equivalents, segregated cash, and restricted cash, end of the period	$ 8,646	$ 10,069	8,646	10,069	$ 9,346
Supplemental disclosures:
Cash paid for interest			8	6
Cash paid for income taxes, net of refund received			$ 6	$ 2