COMMON STOCK - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended			9 Months Ended		12 Months Ended
	Jan. 15, 2019	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Par value of common stock (in dollars per share)		$ 1.25			$ 1.25
Net excess over par value recorded in premium on common stock					$ 183.4
Shares of common stock outstanding (in shares)		92,392,876			92,392,876		85,506,218
Number of shares issued (in shares)					6,779,661
Interest rate		7.50%			7.50%
Quarterly contract adjustment payments, percent							3.55%
Incremental shares included in diluted EPS calculation (in shares)		129,979	1,245,564		116,904	742,313
Dividends per share (in usd per share)		$ 0.29	$ 0.28		$ 0.87	$ 0.84
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Par value of common stock (in dollars per share)				$ 1.25
Shares available for issuance (in shares)				12,669,491
Price per share (in dollars per share)				$ 29.50
Number of shares issued (in shares)				5,889,830
Consideration received	$ 189.0
Capital Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued (in shares)							5,750,000
Corporate Unit, stated value per share (in dollars per share)							$ 50
Over-Allotment Option | Capital Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued (in shares)							750,000
Bank of America, N.A. | Private Placement | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares held for forward contract (in shares)	6,779,661
Series 2018A, Due 2031
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Interest rate							3.70%
Series 2018A, Due 2031 | Capital Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Interest in notes issued, percent							5.00%
SJG
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Par value of common stock (in dollars per share)		$ 2.50			$ 2.50
Shares of common stock outstanding (in shares)		2,339,139			2,339,139
Interest rate		5.587%			5.587%