FINANCIAL INSTRUMENTS - Fair value of Debt Instruments (Details) - Summit Holdings - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
2022 Senior Notes | Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Senior notes	$ 233,322	$ 233,188
2022 Senior Notes | Fair Value | Estimated fair value (Level 2)
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Senior notes	228,391	215,713
2025 Senior Notes | Carrying Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Senior notes	257,201	257,138
2025 Senior Notes | Fair Value | Estimated fair value (Level 2)
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Senior notes	$ 214,814	$ 168,002