AVAILABLE-FOR-SALE SECURITIES AND FAIR VALUE MEASUREMENTS - Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Recurring basis
Assets
Total assets measured at estimated fair value	$ 108,974	$ 122,709
Recurring basis | U.S. government securities
Assets
Total assets measured at estimated fair value	29,733	9,942
Recurring basis | U.S. government agencies
Assets
Total assets measured at estimated fair value	4,971	9,985
Recurring basis | U.S. corporate notes
Assets
Total assets measured at estimated fair value	2,875	10,866
Recurring basis | U.S. commercial paper
Assets
Total assets measured at estimated fair value	22,037	29,945
Recurring basis | Money market funds
Assets
Total assets measured at estimated fair value	49,358	61,971
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1
Assets
Total assets measured at estimated fair value	49,358	61,971
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	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	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	49,358	61,971
Recurring basis | Significant Other Observable Inputs, Level 2
Assets
Total assets measured at estimated fair value	59,616	60,738
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Assets
Total assets measured at estimated fair value	29,733	9,942
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agencies
Assets
Total assets measured at estimated fair value	4,971	9,985
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Assets
Total assets measured at estimated fair value	2,875	10,866
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	22,037	29,945
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	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. corporate notes
Assets
Total assets measured at estimated fair value	0	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 securities
Debt
Term B Loan	13,750	243,750
2023 Notes	258,918	241,259
2025 Notes	230,692	205,975
Total fair value of liabilities	503,360	690,984
Nonrecurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Debt securities
Debt
Term B Loan	0	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 securities
Debt
Term B Loan	13,750	243,750
2023 Notes	258,918	241,259
2025 Notes	230,692	205,975
Total fair value of liabilities	503,360	690,984
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Debt securities
Debt
Term B Loan	0	0
2023 Notes	0	0
2025 Notes	0	0
Total fair value of liabilities	$ 0	$ 0