LONG-TERM DEBT / INTEREST EXPENSE - Components of Convertible Debt (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Liability component
Net carrying amount	$ 1,251,686,000	$ 1,297,023,000
Convertible Senior Notes | 1.5% Convertible Senior Notes, Due 2020
Debt Instrument
Interest rate	1.50%
Liability component
Principal	$ 188,565,000.000	188,565,000
Conversion feature	(14,822,000)	(18,065,000)
Discount related to debt issuance costs	(2,042,000)	(2,512,000)
Net carrying amount	$ 171,701,000	$ 167,988,000