FINANCIAL INSTRUMENTS - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Assets:
Cash equivalents	$ 258,108,000
Marketable securities	0	$ 24,947,000
Total		330,151,000
Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	60,802,000
Total		137,359,000
Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	197,306,000
Total		192,792,000
Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0
Total		0
Treasury discount notes
Assets:
Marketable securities		24,947,000
Treasury discount notes | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Marketable securities		0
Treasury discount notes | Significant Other Observable Inputs (Level 2)
Assets:
Marketable securities		24,947,000
Treasury discount notes | Significant Unobservable Inputs (Level 3)
Assets:
Marketable securities		0
Money market funds
Assets:
Cash equivalents	60,802,000	137,359,000
Money market funds | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	60,802,000	137,359,000
Money market funds | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0	0
Money market funds | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Treasury discount notes
Assets:
Cash equivalents	147,339,000	99,914,000
Treasury discount notes | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	0	0
Treasury discount notes | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	147,339,000	99,914,000
Treasury discount notes | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Commercial paper
Assets:
Cash equivalents	34,967,000	52,931,000
Commercial paper | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	0	0
Commercial paper | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	34,967,000	52,931,000
Commercial paper | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Time deposits
Assets:
Cash equivalents	15,000,000	15,000,000
Time deposits | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	0	0
Time deposits | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	15,000,000	15,000,000
Time deposits | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$ 0	$ 0