FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Changes in Measurements (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 16,061	$ 3,110
Settlements	(5,516)	9,487
Balance at end of period	8,277	16,586
Other Changes in Fair Value from Continuing and New Contracts, Net	(2,300)	3,989
SJG
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	4,928	2,052
Settlements	(4,928)	(2,052)
Balance at end of period	1,706	(6)
Other Changes in Fair Value from Continuing and New Contracts, Net	$ 1,706	$ (6)