Segment Information - Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Gain on Commodity Derivative Instruments	$ 221,581	$ 25,660	$ 26,205	$ 60,747
Other Operating Income	2,923	8,534	6,120	19,244
Total Revenue	605,032	402,124	883,463	897,853
Earnings (Loss) Before Income Tax	233,485	30,292	157,275	789,532
Segment Assets	9,146,697	8,207,758	9,146,697	8,207,758	$ 8,592,170
Depreciation, Depletion and Amortization	128,999	119,087	254,159	243,754
Capital Expenditures	329,227	264,174	628,365	496,659
Equity in earnings of unconsolidated affiliates			1,030	3,447
NJR Energy Services Company
Segment Reporting Information [Line Items]
Total Revenue		36,849	94,447	115,845
Direct Energy Business Marketing LLC
Segment Reporting Information [Line Items]
Total Revenue	48,476	34,644	116,245	92,571
Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	380,528	367,930	851,138	817,862
Earnings (Loss) Before Income Tax	232,674	69,920	158,828	205,264
Equity in earnings of unconsolidated affiliates	527	1,669	1,030	3,447
Investments in unconsolidated equity affiliates	17,637	22,347	17,637	22,347
Operating Segments \| Marcellus Shale
Segment Reporting Information [Line Items]
Gain on Commodity Derivative Instruments	6,831	9,128	(20,627)	(385)
Other Operating Income	0	0	0	0
Total Revenue	244,088	187,433	509,887	382,936
Earnings (Loss) Before Income Tax	50,381	47,273	137,312	91,515
Operating Segments \| Utica Shale
Segment Reporting Information [Line Items]
Gain on Commodity Derivative Instruments	2,747	5,371	(6,854)	897
Other Operating Income	0	0	0	0
Total Revenue	67,960	113,129	151,310	238,977
Earnings (Loss) Before Income Tax	17,941	46,486	49,945	102,593
Operating Segments \| Coalbed Methane
Segment Reporting Information [Line Items]
Gain on Commodity Derivative Instruments	1,085	2,155	(3,217)	(275)
Other Operating Income	0	0	0	0
Total Revenue	40,723	49,037	86,256	104,108
Earnings (Loss) Before Income Tax	7,603	12,405	20,502	25,522
Operating Segments \| Other Gas
Segment Reporting Information [Line Items]
Gain on Commodity Derivative Instruments	210,918	9,006	56,903	60,510
Other Operating Income	3,038	8,595	6,296	19,386
Total Revenue	233,481	29,103	98,848	122,237
Earnings (Loss) Before Income Tax	112,335	(64,039)	(125,929)	(77,695)
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Gain 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	811	(39,628)	(1,553)	584,268
Segment Assets	146,203	333,372	146,203	333,372
Depreciation, Depletion and Amortization	0	0	0	0
Capital Expenditures	0	0	0	0
Intersegment Eliminations
Segment Reporting Information [Line Items]
Gain on Commodity Derivative Instruments	0	0	0	0
Other Operating Income	(115)	(61)	(176)	(142)
Total Revenue	(59,670)	(37,903)	(113,857)	(75,908)
Earnings (Loss) Before Income Tax	0	0	0	0
Segment Assets	(3,041)	(11,166)	(3,041)	(11,166)
Depreciation, Depletion and Amortization		0	0	0
Capital Expenditures	0	0	0	0
E&P \| Operating Segments
Segment Reporting Information [Line Items]
Gain on Commodity Derivative Instruments	221,581	25,660	26,205	60,747
Other Operating Income	3,038	8,595	6,296	19,386
Total Revenue	586,252	378,702	846,301	848,258
Earnings (Loss) Before Income Tax	188,260	42,125	81,830	141,935
Segment Assets	6,887,043	6,055,545	6,887,043	6,055,545
Depreciation, Depletion and Amortization	120,705	111,125	237,780	226,991
Capital Expenditures	224,684	238,889	448,475	455,397
CNXM \| Operating Segments
Segment Reporting Information [Line Items]
Gain on Commodity Derivative Instruments	0	0	0	0
Other Operating Income	0	0	0	0
Total Revenue	78,450	61,325	151,019	125,503
Segment Assets	2,116,492	1,830,007	2,116,492	1,830,007
Depreciation, Depletion and Amortization	8,294	7,962	16,379	16,763
Capital Expenditures	104,543	25,285	179,890	41,262
Oil and Gas
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	342,865	334,517	778,811	740,140
Oil and Gas \| Operating Segments \| Marcellus Shale
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	237,257	178,305	530,514	383,321
Oil and Gas \| Operating Segments \| Utica Shale
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	65,213	107,758	158,164	238,080
Oil and Gas \| Operating Segments \| Coalbed Methane
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	39,638	46,882	89,473	104,383
Oil and Gas \| Operating Segments \| Other Gas
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	757	1,572	660	14,356
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	342,865	334,517	778,811	740,140
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	18,768	9,930	34,989	27,985
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	18,768	9,930	34,989	27,985
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	18,768	9,930	34,989	27,985
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	18,895	23,483	37,338	49,737
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	(59,555)	(37,842)	(113,681)	(75,766)
Midstream Revenue \| E&P \| Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	$ 0	$ 0	$ 0	$ 0

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