DEBT - Components of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Total long-term debt	$ 1,392,986	$ 1,257,731
SMP Holdings
Line of Credit Facility [Line Items]
Total long-term debt	1,392,986	1,257,731
SMP Holdings | Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Revolving credit facility	600,000	466,000
SMP Holdings | Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Unsecured notes	300,000	300,000
Less unamortized debt issuance costs	(1,801)	(2,362)
SMP Holdings | Senior Notes Due April 2025
Line of Credit Facility [Line Items]
Unsecured notes	500,000	500,000
Less unamortized debt issuance costs	$ (5,213)	$ (5,907)