FAIR VALUE MEASUREMENTS AND FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Marketable securities	$ 119,746	$ 4,995
Marketable equity security	664	0
Total	1,282,446	1,167,397
Liabilities:
Contingent consideration arrangements	(1,910)	(2,647)
Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total	889,851	880,882
Liabilities:
Contingent consideration arrangements	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Total	392,595	286,515
Liabilities:
Contingent consideration arrangements	0	0
Significant Unobservable Inputs (Level 3)
Assets:
Total	0	0
Liabilities:
Contingent consideration arrangements	(1,910)	(2,647)
Money market funds
Assets:
Cash equivalents	726,865	780,425
Money market funds | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	726,865	780,425
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
Commercial paper
Assets:
Cash equivalents	191,852	215,325
Marketable securities	19,945	4,995
Commercial paper | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	0	0
Marketable securities	0	0
Commercial paper | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	191,852	215,325
Marketable securities	19,945	4,995
Commercial paper | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Marketable securities	0	0
Treasury discount notes
Assets:
Cash equivalents	162,322	100,457
Marketable securities	99,801
Treasury discount notes | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	162,322	100,457
Marketable securities	0
Treasury discount notes | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0	0
Marketable securities	99,801
Treasury discount notes | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Marketable securities	0
Time deposits
Assets:
Cash equivalents	80,037	60,000
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	80,037	60,000
Time deposits | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Certificates of deposit
Assets:
Cash equivalents	960	6,195
Certificates of deposit | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	0	0
Certificates of deposit | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	960	6,195
Certificates of deposit | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	$ 0
Marketable equity security
Assets:
Marketable equity security	664
Marketable equity security | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Marketable equity security	664
Marketable equity security | Significant Other Observable Inputs (Level 2)
Assets:
Marketable equity security	0
Marketable equity security | Significant Unobservable Inputs (Level 3)
Assets:
Marketable equity security	$ 0