FAIR VALUE MEASUREMENTS - FAIR VALUE OF COMPANY'S FINANCIAL ASSETS AND LIABILITIES REMEASURED ON A RECURRING BASIS (Details) - USD ($)	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfers Of Financial Instruments Between Levels	$ 0
Fair value measurements recurring basis | Money market funds
Financial Assets:
Total financial assets	219,296,000	$ 81,878,000
Fair value measurements recurring basis | Money market funds | Level 1
Financial Assets:
Total financial assets	219,296,000	81,878,000
Fair value measurements recurring basis | Money market funds | Level 2
Financial Assets:
Total financial assets	0	0
Fair value measurements recurring basis | Money market funds | Level 3
Financial Assets:
Total financial assets	0	$ 0
Fair Value, Measurements, Nonrecurring [Member]
Financial Assets:
Acquisition-related contingent consideration	5,142,000
Fair Value, Measurements, Nonrecurring [Member] | Level 1
Financial Assets:
Acquisition-related contingent consideration	0
Fair Value, Measurements, Nonrecurring [Member] | Level 2
Financial Assets:
Acquisition-related contingent consideration	0
Fair Value, Measurements, Nonrecurring [Member] | Level 3
Financial Assets:
Acquisition-related contingent consideration	$ 5,142,000