FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Changes in Measurements (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 3,110	$ 9,035
Other Changes in Fair Value from Continuing and New Contracts, Net	3,989	(988)
Settlements	9,487	8,187
Balance at end of period	16,586	16,234
SJG
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	2,052	926
Other Changes in Fair Value from Continuing and New Contracts, Net	(6)	511
Settlements	(2,052)	(926)
Balance at end of period	$ (6)	$ 511