COMMITMENTS AND CONTINGENCIES - Leases (Details) $ in Thousands		1 Months Ended			6 Months Ended	12 Months Ended
	Feb. 28, 2018 ft² $ / ft² period	Feb. 28, 2019 sublease	Sep. 30, 2018 sublease	Apr. 30, 2012 ft²	Feb. 28, 2019 sublease	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2013
Leases
Aggregate rent payable						$ 10,292
Number of subleases | sublease			1
Enterprise FM Trust
Leases
Aggregate rent payable						800
Enterprise FM Trust | Minimum
Leases
Lease terms									18 months
Enterprise FM Trust | Maximum
Leases
Lease terms									36 months
Lease of cars
Leases
Rent expense						800	$ 3,200	$ 3,200
Lake Forest Illinois Building
Leases
Square feet agreed to lease | ft²	31,000
Aggregate rent payable						3,000
Lease terms	5 years 6 months
Number of renewal periods | period	1
Renewal terms	5 years
Written notice period (no later than)	12 months
Initial annual base rent | $ / ft²	18.00
Annual increase in base rent | $ / ft²	0.50
Tenant improvement allowance | $ / ft²	28.00
New lease Newark
Leases
Square feet agreed to lease | ft²				52,500
Additional square feet obligated lease | ft²				8,000
Aggregate rent payable						6,400
Liability for deferred tenant improvement allowance						3,600
Newark and Lake Forest
Leases
Deferred rent						1,600	1,400
Newark and Lake Forest | Office and laboratory
Leases
Rent expense						$ 600	$ 300	$ 600
Subsequent Event
Leases
Number of subleases | sublease		1			2