FINANCIAL INSTRUMENTS - Schedule of Estimated Fair Values and Carrying Values of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Estimated fair values of long-term debt	$ 3,604,517	$ 3,152,224
Carrying amounts of long-term debt, including current maturities	3,261,945	2,919,201
Net of:
Unamortized debt issuance costs	39,465	29,574
Unamortized debt discounts	5,157	5,224
Finance lease, lease liability	5,600	3,100
SJG
Debt Instrument [Line Items]
Estimated fair values of long-term debt	1,171,515	1,197,052
Carrying amounts of long-term debt, including current maturities	1,049,551	1,069,089
Net of:
Unamortized debt issuance costs	$ 8,895	$ 9,357