SUBSEQUENT EVENTS - GSR Agreements (Details) $ / shares in Units, $ in Thousands	May 06, 2019 USD ($)	Aug. 09, 2018 USD ($) $ / shares $ / token shares	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Subsequent Event [Line Items]
Post-money valuation			$ 35,607	$ 25,529
GSR Capital | Subsequent Event
Subsequent Event [Line Items]
Aggregate token offering price		$ 160,000
Sale of tokens (in usd per token) | $ / token		6.67
Aggregate price of term sheet		$ 104,500
Number of shares issued, up to (in shares) | shares		3,100,000
Sale of stock (in usd per share) | $ / shares		$ 33.72
Aggregate offering price		$ 270,000
tZero.com, Inc. | Subsequent Event
Subsequent Event [Line Items]
Post-money valuation		$ 1,500,000
tZero.com, Inc. | GSR Capital | Subsequent Event
Subsequent Event [Line Items]
Cash portion of acquisition price	$ 30,000
Maximum ownership percentage		18.00%