LINES OF CREDIT AND SHORT-TERM BORROWINGS - Schedule of Lines of Credit (Weighted Average Interest Rates, Average Borrowings, and Maximum Amounts Outstanding) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Line of Credit Facility [Line Items]
Maximum amounts outstanding	$ 452.9	$ 872.2
SJG
Line of Credit Facility [Line Items]
Maximum amounts outstanding	$ 47.5	$ 171.7
Line of Credit
Line of Credit Facility [Line Items]
Weighted average interest rate on borrowings	1.40%	2.02%
Average borrowings outstanding, not including LOC	$ 366.5	$ 774.0
Line of Credit | SJG
Line of Credit Facility [Line Items]
Weighted average interest rate on borrowings	0.19%	1.76%
Average borrowings outstanding, not including LOC	$ 21.2	$ 152.5