COMMITMENTS AND CONTINGENCIES - NARRATIVE (Details) $ in Thousands				1 Months Ended
	Jun. 30, 2021 USD ($)	Jan. 28, 2021 lawsuit	Dec. 31, 2020 USD ($) lawsuit	Sep. 30, 2021 USD ($) plaintiff option	Jul. 31, 2021 lawsuit	Jan. 31, 2021 tranche	Dec. 31, 2020 USD ($) tranche	Mar. 31, 2020 plaintiff lawsuit
Lessee, Lease, Description [Line Items]
Number of renewal options (or more) | option				1
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]			Other non-current assets	Other non-current assets			Other non-current assets
Operating lease right-of-use assets			$ 49,200	$ 158,100			$ 49,200
Total lease liabilities			$ 54,100	$ 154,537			$ 54,100
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]			Other current liabilities	Other current liabilities			Other current liabilities
Operating lease liabilities			$ 6,100	$ 20,200			$ 6,100
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]			Other non-current liabilities	Other non-current liabilities			Other non-current liabilities
Operating lease liabilities, net of current portion			$ 48,000	$ 134,300			$ 48,000
Accrued for legal and regulatory contingencies			$ 39,500	$ 33,100			$ 39,500
Minimum
Lessee, Lease, Description [Line Items]
Remaining lease term				1 year
Maximum
Lessee, Lease, Description [Line Items]
Remaining lease term				12 years
Massachusetts Securities Law Violations
Lessee, Lease, Description [Line Items]
Number of lawsuits | tranche							3
Best Execution, Payment for Order Flow, and Source of Revenue Matters | Unfavorable Regulatory Action
Lessee, Lease, Description [Line Items]
Fine, amount							$ 65,000
Penalty paid in cash							$ 65,000
Putative Securities Fraud Class Action Lawsuit
Lessee, Lease, Description [Line Items]
Number of lawsuits | lawsuit			7
Putative Class Actions
Lessee, Lease, Description [Line Items]
Number of lawsuits | lawsuit		55						15
Number of customers | tranche						2,000
Individual Action
Lessee, Lease, Description [Line Items]
Number of lawsuits | lawsuit		4						1
March 2020 Outages
Lessee, Lease, Description [Line Items]
Number of customers | plaintiff				400				1,600
FINRA Multi-Matter Settlement | Unfavorable Regulatory Action
Lessee, Lease, Description [Line Items]
Accrued for legal and regulatory contingencies	$ 57,000
Fine, amount	57,000
FINRA Multi-Matter Settlement | Unfavorable Regulatory Action, Customer Restitution
Lessee, Lease, Description [Line Items]
Fine, amount	$ 12,600
Consolidated Claims
Lessee, Lease, Description [Line Items]
Number of lawsuits | lawsuit					2