EMPLOYEE BENEFIT PLANS - Amounts Recognized in the Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Employee benefit plans
Noncurrent assets	$ 0	$ 0
Accrued benefit costs	(104)	62
Accrued benefit liability	(117,145)	(238,454)
Regulatory assets	58,101	187,930
Regulatory liabilities	0	0
Net amount recognized	(59,148)	(50,462)
Other Benefits
Employee benefit plans
Noncurrent assets	16,399	0
Accrued benefit costs	(1,782)	(2,441)
Accrued benefit liability	0	(21,961)
Regulatory assets	1,487	20,392
Regulatory liabilities	(18,472)	0
Net amount recognized	$ (2,368)	$ (4,010)