Segment Information - Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	$ 213,913	$ 18,005	$ 240,118	$ 78,752
Other Operating Income	3,316	3,903	9,436	23,146
Total Revenue	529,997	397,126	1,413,460	1,294,979
Earnings (Loss) Before Income Tax	192,862	203,434	350,137	992,965
Segment Assets	9,286,258	8,209,036	9,286,258	8,209,036	$ 8,592,170
Depreciation, Depletion and Amortization	120,459	119,585	374,619	363,338
Capital Expenditures	336,137	297,465	964,502	794,124
Equity in earnings of unconsolidated affiliates			1,703	4,688
NJR Energy Services Company
Segment Reporting Information [Line Items]
Total Revenue		42,901	114,440	158,746
Direct Energy Business Marketing LLC
Segment Reporting Information [Line Items]
Total Revenue	39,092		155,337
Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	312,768	375,218	1,163,906	1,193,081
Earnings (Loss) Before Income Tax	192,971	85,604	351,799	290,868
Equity in earnings of unconsolidated affiliates	673	1,241	1,703	4,688
Investments in unconsolidated equity affiliates	17,110	19,488	17,110	19,488
Operating Segments | Marcellus Shale
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	38,354	1,796	17,727	1,411
Other Operating Income	0	0	0	0
Total Revenue	217,496	209,203	727,383	592,139
Earnings (Loss) Before Income Tax	41,014	64,408	178,326	155,923
Operating Segments | Utica Shale
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	12,628	(151)	5,774	746
Other Operating Income	0	0	0	0
Total Revenue	62,670	87,888	213,980	326,865
Earnings (Loss) Before Income Tax	18,220	41,237	68,165	143,830
Operating Segments | Coalbed Methane
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	6,036	605	2,819	330
Other Operating Income	0	0	0	0
Total Revenue	41,657	49,076	127,913	153,184
Earnings (Loss) Before Income Tax	8,717	9,642	29,219	35,164
Operating Segments | Other Gas
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	156,895	15,755	213,798	76,265
Other Operating Income	3,375	3,969	9,671	23,355
Total Revenue	189,708	31,079	288,556	153,316
Earnings (Loss) Before Income Tax	83,279	(60,856)	(42,650)	(138,551)
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	0	0	0	0
Other Operating Income	0	0	0	0
Total Revenue	0	0	0	0
Earnings (Loss) Before Income Tax	(109)	117,830	(1,662)	702,097
Segment Assets	16,668	82,696	16,668	82,696
Depreciation, Depletion and Amortization	0	0	0	0
Capital Expenditures	0	0	0	0
Intersegment Eliminations
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	0	0	0	0
Other Operating Income	(59)	(66)	(235)	(209)
Total Revenue	(55,795)	(41,492)	(169,652)	(117,400)
Earnings (Loss) Before Income Tax	0	0	0	0
Segment Assets	3,735	(12,926)	3,735	(12,926)
Depreciation, Depletion and Amortization	0	0	0	0
Capital Expenditures	0	0	0	0
E&P | Operating Segments
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	213,913	18,005	240,118	78,752
Other Operating Income	3,375	3,969	9,671	23,355
Total Revenue	511,531	377,246	1,357,832	1,225,504
Earnings (Loss) Before Income Tax	151,230	54,431	233,060	196,366
Segment Assets	7,087,782	6,256,132	7,087,782	6,256,132
Depreciation, Depletion and Amortization	111,839	111,844	349,620	338,834
Capital Expenditures	267,689	253,263	716,164	708,660
CNXM | Operating Segments
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	0	0	0	0
Other Operating Income	0	0	0	0
Total Revenue	74,261	61,372	225,280	186,875
Earnings (Loss) Before Income Tax	41,741	31,173	118,739	94,502
Segment Assets	2,178,073	1,883,134	2,178,073	1,883,134
Depreciation, Depletion and Amortization	8,620	7,741	24,999	24,504
Capital Expenditures	68,448	44,202	248,338	85,464
Oil and Gas
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	265,051	344,712	1,043,862	1,084,851
Oil and Gas | Operating Segments | Marcellus Shale
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	179,142	207,407	709,656	590,728
Oil and Gas | Operating Segments | Utica Shale
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	50,042	88,039	208,206	326,119
Oil and Gas | Operating Segments | Coalbed Methane
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	35,621	48,471	125,094	152,854
Oil and Gas | Operating Segments | Other Gas
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	246	795	906	15,150
Oil and Gas | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Oil and Gas | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Oil and Gas | E&P | Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	265,051	344,712	1,043,862	1,084,851
Oil and Gas | CNXM | Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Purchased Gas Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	29,192	10,560	64,181	38,546
Purchased Gas Revenue | Operating Segments | Marcellus Shale
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Purchased Gas Revenue | Operating Segments | Utica Shale
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Purchased Gas Revenue | Operating Segments | Coalbed Methane
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Purchased Gas Revenue | Operating Segments | Other Gas
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	29,192	10,560	64,181	38,546
Purchased Gas Revenue | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Purchased Gas Revenue | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Purchased Gas Revenue | E&P | Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	29,192	10,560	64,181	38,546
Purchased Gas Revenue | CNXM | Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Midstream Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	18,525	19,946	55,863	69,684
Midstream Revenue | Operating Segments | Marcellus Shale
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Midstream Revenue | Operating Segments | Utica Shale
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Midstream Revenue | Operating Segments | Coalbed Methane
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Midstream Revenue | Operating Segments | Other Gas
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Midstream Revenue | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Midstream Revenue | Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	(55,736)	(41,426)	(169,417)	(117,191)
Midstream Revenue | E&P | Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Midstream Acquisition | CNXM | Operating Segments
Segment Reporting Information [Line Items]
Earnings (Loss) Before Income Tax	41,741	31,173	118,739	94,502
Midstream Acquisition | Midstream Revenue | CNXM | Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	$ 74,261	$ 61,372	$ 225,280	$ 186,875