INCOME TAXES - Provision for Income Taxes Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income tax expense reconciliation (percent)
Expected federal income tax	21.00%	21.00%	35.00%
State income taxes	(26.10%)	8.00%	8.00%
Permanent items	(6.50%)	1.10%	0.30%
Research and development tax credits	(17.10%)	1.60%	(2.60%)
Excess tax benefits and stock-based compensation	(13.90%)	(1.00%)	(0.70%)
Acquisition-related tax adjustments	3.20%	(2.10%)	(1.40%)
Enactment of the Tax Act	0.00%	0.00%	(47.40%)
Other	7.10%	(1.70%)	2.00%
Income tax expense	(32.30%)	26.90%	(6.80%)
Income tax expense reconciliation
Expected federal income tax	$ 1,234	$ (11,247)	$ 11,406
State income taxes	(1,536)	(4,270)	2,606
Permanent items	(384)	(614)	88
Research and development tax credits	(1,003)	(850)	(850)
Excess tax benefits and stock-based compensation	(815)	559	(243)
Acquisition-related tax adjustments	189	1,144	(445)
Enactment of the Tax Act	0	0	(15,461)
Other	418	885	668
Total benefit from income taxes	(1,897)	$ (14,393)	$ (2,231)
State and Local Jurisdiction
Income tax expense reconciliation
Enactment of the Tax Act	$ 2,000