COMMITMENTS, CONTINGENCIES AND REGULATORY MATTERS - Leases (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
Right-to-use assets	$ 20,303		$ 20,303		$ 29,074	$ 42,100
Lease obligation liabilities	21,923		21,923			45,500
Lease obligation liabilities, current	8,186		8,186		11,398	16,700
Lease obligation liabilities, noncurrent	13,737		13,737		$ 19,707	28,800
Reduction in reduced accrued rent and lease incentives						$ 3,400
Sublease income	$ 300	$ 500	$ 1,000	$ 1,300
Lease term, not commenced	5 years		5 years
Minimum
Lessee, Lease, Description [Line Items]
Lease term	1 year		1 year
Maximum
Lessee, Lease, Description [Line Items]
Lease term	6 years		6 years