LONG-TERM DEBT / INTEREST EXPENSE - Schedule of Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument
Interest income and other	$ 627	$ (56)	$ (123)
Interest expense, net	64,221	60,443	50,936
Senior Credit Facility
Debt Instrument
Amortization of discount and debt issuance costs	2,413	2,444	2,583
Interest expense, loss on debt financing, and amortization expense	48,575	45,954	45,535
Revolving Credit Facility | Senior Credit Facility
Debt Instrument
Interest expense, excluding amortization of conversion feature, discount and debt issuance costs	4,685	4,689	805
Term Loan Facility coupon | Senior Credit Facility
Debt Instrument
Interest expense, excluding amortization of conversion feature, discount and debt issuance costs	41,477	38,821	42,147
Notes | Notes
Debt Instrument
Interest expense, excluding amortization of conversion feature, discount and debt issuance costs	4,272	4,313	1,702
Amortization of discount and debt issuance costs	1,251	1,140	412
Amortization of conversion feature	9,496	9,092	3,410
Interest expense, loss on debt financing, and amortization expense	$ 15,019	$ 14,545	$ 5,524