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	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Line of Credit Facility [Line Items]
Maximum amounts outstanding	$ 872.2	$ 882.7
SJG
Line of Credit Facility [Line Items]
Maximum amounts outstanding	$ 187.0	$ 175.3
Line of Credit
Line of Credit Facility [Line Items]
Weighted average interest rate on borrowings	1.26%	3.00%
Average borrowings outstanding, not including LOC	$ 516.8	$ 487.8
Line of Credit | SJG
Line of Credit Facility [Line Items]
Weighted average interest rate on borrowings	0.23%	2.39%
Average borrowings outstanding, not including LOC	$ 149.0	$ 96.2