FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Changes in Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 18,361	$ 17,401	$ 3,110	$ 9,035
Other Changes in Fair Value from Continuing and New Contracts, Net	(4,426)	1,352	5,778	8,346
Transfers out of Level 3 (A)		(206)		(954)
Settlements	(3,631)	(4,781)	1,416	(2,661)
Balance at end of period	10,304	13,766	10,304	13,766
SJG
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	5,997	6,933	2,052	926
Other Changes in Fair Value from Continuing and New Contracts, Net	619	(1,603)	6,616	5,330
Transfers out of Level 3 (A)		(206)		(206)
Settlements	0	0	(2,052)	(926)
Balance at end of period	$ 6,616	$ 5,124	$ 6,616	$ 5,124