COMMON STOCK - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands					1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Apr. 15, 2021	Apr. 01, 2021	Mar. 25, 2021	Mar. 22, 2021	Mar. 31, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2018	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Par value of common stock (in dollars per share)						$ 1.25		$ 1.25
Shares of common stock outstanding (in shares)						112,445,978		112,445,978			100,591,940
Number of shares issued (in shares)				1,899,859
Gross proceeds from public offering				$ 42,300
Net proceeds from public offering, after underwriting discounts and commissions				$ 40,600
Incremental shares included in diluted EPS calculation (in shares)						1,843,819	121,976	1,486,997	116,527
Corporate Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Gross proceeds from public offering					$ 300,000
Net proceeds from public offering, after underwriting discounts and commissions					291,000
Public Stock Offering
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Par value of common stock (in dollars per share)				$ 1.25
Number of shares offered (in shares)				10,250,000
Price per share (in dollars per share)				$ 22.25
Number of shares issued (in shares)				362,359
Quarterly contract adjustment payments, percent										3.55%
Public Stock Offering | Corporate Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Price per share (in dollars per share)	$ 50.00
Number of shares issued (in shares)				6,000,000						5,750,000
Gross proceeds from public offering				$ 300,000
Net proceeds from public offering, after underwriting discounts and commissions				$ 291,000
Corporate unit, stated value per share (in dollars per share)				$ 50		$ 50		$ 50		$ 50
Quarterly contract adjustment payments, percent				7.10%
Quarterly cash distribution per share (in dollars per share)		$ 0.90625
Public Stock Offering | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued (in shares)	9,800,000
Net proceeds from public offering, after underwriting discounts and commissions	$ 287,500
Shares to be received for each stock purchase contract held (in shares)	1.7035	1.7035								1.6949
Private Placement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares held for forward contract (in shares)				9,887,641
Over-Allotment Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Price per share (in dollars per share)			$ 22.25
Number of shares issued (in shares)			1,537,500
Over-Allotment Option | Corporate Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares offered (in shares)		900,000
Number of shares issued (in shares)		700,000								750,000
Gross proceeds from public offering		$ 35,000
Net proceeds from public offering, after underwriting discounts and commissions		$ 34,000
2018 Series A Remarketable Junior Notes due 2031
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Interest rate			5.02%							3.70%
Subordinated notes			$ 287,500		$ 287,500
Effective interest rate (as a percent)						0.05%		0.05%
2018 Series A Remarketable Junior Notes due 2031 | Public Stock Offering | Corporate Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Interest in notes issued, percent										5.00%
2021 Series B Remarketable Junior Subordinated Notes due 2029
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Interest rate				1.65%
Effective interest rate (as a percent)						2.10%		2.10%
2021 Series B Remarketable Junior Subordinated Notes due 2029 | Public Stock Offering | Corporate Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Interest in notes issued, percent				5.00%
Convertible
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Interest expense, coupon						$ 5,100	$ 2,700	$ 7,800	$ 5,300
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
Additional Investment by Shareholder						$ 0	$ 9,500	$ 0	$ 9,500