Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 323,019	$ 357,200	$ 1,258,301	$ 1,399,991
Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	94,502	153,681	385,065	486,670
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0	0	0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	36,726	5,194	67,663	(18,817)
Revenue	131,228	158,875	452,728	467,853
Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	77,813	68,967	229,278	218,375
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0	0	0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	0	0	0	0
Revenue	77,813	68,967	229,278	218,375
Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	33,299	32,875	103,355	91,933
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0	0	0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	0	0	0	0
Revenue	33,299	32,875	103,355	91,933
Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	126,545	132,456	570,580	660,136
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	492	465	7,775	(1,528)
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	0	0	0	0
Revenue	127,037	132,921	578,355	658,608
All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	20,164	23,071	92,647	132,200
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0	0	0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	(3,537)	(201)	(8,604)	3,461
Revenue	16,627	22,870	84,043	135,661
Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(62,985)	(59,308)	(189,458)	(172,439)
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0	0	0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	0	0	0	0
Revenue	(62,985)	(59,308)	(189,458)	(172,439)
Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	289,338	351,742	1,191,467	1,416,875
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	492	465	7,775	(1,528)
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	33,189	4,993	59,059	(15,356)
Revenue	323,019	357,200	1,258,301	1,399,991
Production of Natural Gas [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	76,831	113,975	297,481	371,710
Production of Natural Gas [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Natural Gas [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Natural Gas [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Natural Gas [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Natural Gas [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Natural Gas [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	76,831	113,975	297,481	371,710
Production of Crude Oil [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	17,018	38,823	84,949	111,256
Production of Crude Oil [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Crude Oil [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Crude Oil [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Crude Oil [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Crude Oil [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Production of Crude Oil [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	17,018	38,823	84,949	111,256
Natural Gas Processing [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	435	731	1,838	2,676
Natural Gas Processing [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Processing [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Processing [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Processing [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Processing [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Processing [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	435	731	1,838	2,676
Natural Gas Gathering Services [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Gathering Services [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Gathering Services [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	33,299	32,875	103,355	91,931
Natural Gas Gathering Services [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Gathering Services [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Gathering Services [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(33,299)	(32,875)	(103,355)	(91,931)
Natural Gas Gathering Services [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Transportation Service [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Transportation Service [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	57,563	50,001	169,469	158,376
Natural Gas Transportation Service [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Transportation Service [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	22,473	23,010	95,112	103,723
Natural Gas Transportation Service [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Transportation Service [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(20,445)	(17,672)	(59,408)	(54,556)
Natural Gas Transportation Service [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	59,591	55,339	205,173	207,543
Natural Gas Storage Service [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Storage Service [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	20,016	18,598	58,966	56,887
Natural Gas Storage Service [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Storage Service [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Storage Service [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Storage Service [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(8,802)	(8,060)	(25,881)	(24,367)
Natural Gas Storage Service [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	11,214	10,538	33,085	32,520
Natural Gas Residential Sales [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Residential Sales [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Residential Sales [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Residential Sales [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	93,853	96,146	423,547	492,267
Natural Gas Residential Sales [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Residential Sales [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Residential Sales [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	93,853	96,146	423,547	492,267
Natural Gas Commercial Sales [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Commercial Sales [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Commercial Sales [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Commercial Sales [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	10,264	12,107	56,401	68,408
Natural Gas Commercial Sales [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Commercial Sales [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Commercial Sales [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	10,264	12,107	56,401	68,408
Natural Gas Industrial Sales [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Industrial Sales [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Industrial Sales [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Industrial Sales [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	616	1,032	3,029	4,400
Natural Gas Industrial Sales [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Industrial Sales [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Industrial Sales [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	616	1,032	3,029	4,400
Natural Gas Marketing [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Marketing [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Marketing [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Marketing [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	0
Natural Gas Marketing [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	19,149	22,212	89,662	130,015
Natural Gas Marketing [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(341)	(681)	(598)	(1,056)
Natural Gas Marketing [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	18,808	21,531	89,064	128,959
Other [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	218	152	797	1,028
Other [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	234	368	843	3,112
Other [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0	0	0	2
Other [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(661)	161	(7,509)	(8,662)
Other [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,015	859	2,985	2,185
Other [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(98)	(20)	(216)	(529)
Other [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	$ 708	$ 1,520	$ (3,100)	$ (2,864)