INVESTMENTS IN EQUITY INVESTEES - FINANCIAL INFORMATION FOR STECKMAN RIDGE AND PENNEAST (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Assets
Current assets	$ 634,274	$ 622,332
Liabilities
Current liabilities	1,051,410	533,722
Noncurrent liabilities	877,842	879,393
Income Statement [Abstract]
Operating revenues	2,156,613	1,953,668	$ 2,592,045
Net income, as reported	117,890	163,007	123,935
Steckman Ridge
Assets
Current assets	14,786	7,979
Noncurrent assets	202,670	207,051
Liabilities
Current liabilities	9,738	945
Noncurrent liabilities	140,810	140,810
Income Statement [Abstract]
Operating revenues	21,847	28,814	32,087
Gross profit	13,350	20,537	24,051
Income from continuing operations	11,483	16,926	18,944
Net income, as reported	11,483	16,926	18,944
Net income attributable to NJR	5,742	8,463	9,472
PennEast
Assets
Current assets	822	2,829
Noncurrent assets	44,998	446,212
Liabilities
Current liabilities	248	1,761
Noncurrent liabilities	500	500
Income Statement [Abstract]
Operating revenues	0	0	0
Gross profit	0	0	0
Income from continuing operations	(406,305)	34,376	31,689
Net income, as reported	(406,305)	34,376	31,689
Net income attributable to NJR	$ (81,261)	$ 6,875	$ 6,338