Segment Information - Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	$ (168,834)	$ 213,913	$ (116,995)	$ 240,118
Other Revenue and Operating Income	21,155	21,841	60,463	65,299
Total Revenue and Other Operating Income	66,075	529,997	631,275	1,413,460
Total Operating Expense	279,505	298,599	1,385,298	939,234
(Loss) Earnings Before Income Tax	(250,072)	192,862	(867,009)	350,137
Segment Assets	8,129,174	9,286,258	8,129,174	9,286,258	$ 9,060,806
Depreciation, Depletion and Amortization	114,464	120,459	357,174	374,619
Capital Expenditures	108,335	336,137	395,236	964,502
Equity in earnings (loss) of unconsolidated affiliates			(1,025)	1,703
Investments in unconsolidated equity affiliates	15,685	17,110	15,685	17,110
Natural Gas | Commodity Swap
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	(297,059)	188,058	(167,803)	355,759
Natural Gas, NGL and Oil Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	182,213	265,051	609,483	1,043,862
Natural Gas, NGL and Oil Revenue | Direct Energy Business Marketing LLC
Segment Reporting Information [Line Items]
Total Revenue and Other Operating Income	35,645	39,092	115,011	155,337
Natural Gas, NGL and Oil Revenue | NJR Energy Services Company
Segment Reporting Information [Line Items]
Total Revenue and Other Operating Income				114,440
Purchased Gas Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	31,541	29,192	78,324	64,181
Operating Segments
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	230,443	312,767	735,093	1,163,906
Segment Assets	7,195,668	8,143,186	7,195,668	8,143,186
Operating Segments | Shale
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	79,773	50,982	266,488	23,501
Other Revenue and Operating Income	16,689	18,525	47,286	55,863
Total Revenue and Other Operating Income	252,254	298,690	842,687	997,226
Total Operating Expense	165,587	185,452	511,653	586,030
(Loss) Earnings Before Income Tax	86,667	113,238	331,034	411,196
Segment Assets	6,070,356	6,911,677	6,070,356	6,911,677
Depreciation, Depletion and Amortization	94,617	100,545	295,648	316,596
Capital Expenditures	105,988	332,640	385,763	951,427
Operating Segments | Shale | Natural Gas, NGL and Oil Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	155,792	229,183	528,913	917,862
Operating Segments | Shale | Purchased Gas Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Operating Segments | Coalbed Methane
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	10,518	6,036	33,210	2,819
Other Revenue and Operating Income	0	0	0	0
Total Revenue and Other Operating Income	36,185	41,657	112,529	127,913
Total Operating Expense	30,439	32,940	94,113	98,694
(Loss) Earnings Before Income Tax	5,746	8,717	18,416	29,219
Segment Assets	1,125,312	1,231,509	1,125,312	1,231,509
Depreciation, Depletion and Amortization	15,763	17,400	49,903	51,669
Capital Expenditures	2,310	2,196	6,796	7,752
Operating Segments | Coalbed Methane | Natural Gas, NGL and Oil Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	25,667	35,621	79,319	125,094
Operating Segments | Coalbed Methane | Purchased Gas Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Other
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments	(259,125)	156,895	(416,693)	213,798
Other Revenue and Operating Income	4,466	3,316	13,177	9,436
Total Revenue and Other Operating Income	(222,364)	189,650	(323,941)	288,321
Total Operating Expense	83,479	80,207	779,532	254,510
(Loss) Earnings Before Income Tax	(342,485)	70,907	(1,216,459)	(90,278)
Segment Assets	933,506	1,143,072	933,506	1,143,072
Depreciation, Depletion and Amortization	4,084	2,514	11,623	6,354
Capital Expenditures	37	1,301	2,677	5,323
Equity in earnings (loss) of unconsolidated affiliates	396	673	(1,025)	1,703
Other | Natural Gas | Commodity Swap
Segment Reporting Information [Line Items]
Gain (Loss) on Commodity Derivative Instruments			83,997
Other | Natural Gas, NGL and Oil Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	754	247	1,251	906
Other | Purchased Gas Revenue
Segment Reporting Information [Line Items]
Revenue from Contracts with Customers	$ 31,541	$ 29,192	$ 78,324	$ 64,181