AVAILABLE-FOR-SALE SECURITIES AND FAIR VALUE MEASUREMENTS - Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Recurring basis
Assets
Total assets measured at estimated fair value	$ 306,741	$ 108,974
Recurring basis | U.S. government securities
Assets
Total assets measured at estimated fair value	53,834	29,733
Recurring basis | U.S. government agencies
Assets
Total assets measured at estimated fair value		4,971
Recurring basis | U.S. corporate notes
Assets
Total assets measured at estimated fair value		2,875
Recurring basis | U.S. commercial paper
Assets
Total assets measured at estimated fair value	18,915	22,037
Recurring basis | Money market funds
Assets
Total assets measured at estimated fair value	233,992	49,358
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1
Assets
Total assets measured at estimated fair value	233,992	49,358
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. government securities
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. government agencies
Assets
Total assets measured at estimated fair value		0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. corporate notes
Assets
Total assets measured at estimated fair value		0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Money market funds
Assets
Total assets measured at estimated fair value	233,992	49,358
Recurring basis | Significant Other Observable Inputs, Level 2
Assets
Total assets measured at estimated fair value	72,749	59,616
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Assets
Total assets measured at estimated fair value	53,834	29,733
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agencies
Assets
Total assets measured at estimated fair value		4,971
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Assets
Total assets measured at estimated fair value		2,875
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	18,915	22,037
Recurring basis | Significant Other Observable Inputs, Level 2 | Money market funds
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government securities
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government agencies
Assets
Total assets measured at estimated fair value		0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. corporate notes
Assets
Total assets measured at estimated fair value		0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Money market funds
Assets
Total assets measured at estimated fair value	0	0
Nonrecurring basis | Debt
Debt
Term B Loan		13,750
2023 Notes	243,394	258,918
2025 Notes	208,976	230,692
Total fair value of liabilities	452,370	503,360
Nonrecurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Debt
Debt
Term B Loan		0
2023 Notes	0	0
2025 Notes	0	0
Total fair value of liabilities	0	0
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Debt
Debt
Term B Loan		13,750
2023 Notes	243,394	258,918
2025 Notes	208,976	230,692
Total fair value of liabilities	452,370	503,360
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Debt
Debt
Term B Loan		0
2023 Notes	0	0
2025 Notes	0	0
Total fair value of liabilities	$ 0	$ 0