AVAILABLE-FOR-SALE SECURITIES AND FAIR VALUE MEASUREMENTS - Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Recurring basis
Assets
Total assets measured at estimated fair value	$ 122,709	$ 148,813
Recurring basis | U.S. government securities
Assets
Total assets measured at estimated fair value	9,942
Recurring basis | U.S. government agencies
Assets
Total assets measured at estimated fair value	9,985	12,429
Recurring basis | U.S. corporate notes
Assets
Total assets measured at estimated fair value	10,866
Recurring basis | U.S. commercial papers
Assets
Total assets measured at estimated fair value	29,945	72,065
Recurring basis | Money market funds
Assets
Total assets measured at estimated fair value	61,971	64,319
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Assets
Total assets measured at estimated fair value	61,971	64,319
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government securities
Assets
Total assets measured at estimated fair value	0
Recurring basis | Quoted Prices 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 Prices in Active Markets for Identical Assets, Level 1 | U.S. corporate notes
Assets
Total assets measured at estimated fair value	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. commercial papers
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Money market funds
Assets
Total assets measured at estimated fair value	61,971	64,319
Recurring basis | Significant Other Observable Inputs, Level 2
Assets
Total assets measured at estimated fair value	60,738	84,494
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Assets
Total assets measured at estimated fair value	9,942
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agencies
Assets
Total assets measured at estimated fair value	9,985	12,429
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Assets
Total assets measured at estimated fair value	10,866
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial papers
Assets
Total assets measured at estimated fair value	29,945	72,065
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
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
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. commercial papers
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
Liabilities
Term B Loan	243,750
2023 Notes	241,259	202,125
2025 Notes	205,975
2029 Notes		487,189
Total fair value of liabilities	690,984	689,314
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Debt Securities
Liabilities
Term B Loan	0
2023 Notes	0	0
2025 Notes	0
2029 Notes		0
Total fair value of liabilities	0	0
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Debt Securities
Liabilities
Term B Loan	243,750
2023 Notes	241,259	202,125
2025 Notes	205,975
2029 Notes		487,189
Total fair value of liabilities	690,984	689,314
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Debt Securities
Liabilities
Term B Loan	0
2023 Notes	0	0
2025 Notes	0
2029 Notes		0
Total fair value of liabilities	$ 0	$ 0