DEBT - Components of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Total long-term debt	$ 1,147,005	$ 1,051,192
SMP Holdings
Line of Credit Facility [Line Items]
Total long-term debt	1,147,005	1,051,192
SMP Holdings | Senior Secured Revolving Credit Facility Due May 2022
Line of Credit Facility [Line Items]
Revolving credit facility	356,000	261,000
SMP Holdings | Senior Notes Due August 2022
Line of Credit Facility [Line Items]
Unsecured notes	300,000	300,000
Less unamortized debt issuance costs	(2,588)	(2,910)
SMP Holdings | Senior Notes Due April 2025
Line of Credit Facility [Line Items]
Unsecured notes	500,000	500,000
Less unamortized debt issuance costs	$ (6,407)	$ (6,898)