FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Marketable securities	$ 39,954	$ 123,246
Marketable equity security	447	419
Assets, Fair Value Disclosure	1,774,663	1,818,666
Liabilities:
Contingent consideration arrangement	(28,186)	(28,631)
Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Assets, Fair Value Disclosure	734,329	881,234
Liabilities:
Contingent consideration arrangement	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Assets, Fair Value Disclosure	1,040,334	937,432
Liabilities:
Contingent consideration arrangement	0	0
Significant Unobservable Inputs (Level 3)
Assets:
Assets, Fair Value Disclosure	0	0
Liabilities:
Contingent consideration arrangement	(28,186)	(28,631)
Money market funds
Assets:
Cash equivalents	733,882	880,815
Money market funds | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	733,882	880,815
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	771,604	561,733
Marketable securities	24,983	112,291
Treasury discount notes | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	0	0
Marketable securities	0	0
Treasury discount notes | Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	771,604	561,733
Marketable securities	24,983	112,291
Treasury discount notes | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Marketable securities	0	0
Commercial paper
Assets:
Cash equivalents	118,740	162,417
Marketable securities	14,971	10,955
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	118,740	162,417
Marketable securities	14,971	10,955
Commercial paper | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Marketable securities	0	0
Time deposits
Assets:
Cash equivalents	110,036	90,036
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	110,036	90,036
Time deposits | Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Marketable equity security
Assets:
Marketable equity security	447	419
Marketable equity security | Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Marketable equity security	447	419
Marketable equity security | Significant Other Observable Inputs (Level 2)
Assets:
Marketable equity security	0	0
Marketable equity security | Significant Unobservable Inputs (Level 3)
Assets:
Marketable equity security	$ 0	$ 0