Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 287,989	$ 323,019	$ 491,095	$ 444,188	$ 293,341	$ 357,200	$ 552,544	$ 490,247	$ 1,546,291	$ 1,693,332	$ 1,592,668
Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									513,762	635,012
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition									0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition									93,691	(2,272)
Revenue									607,453	632,740
Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									309,604	288,283
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition									0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition									0	0
Revenue									309,604	288,283
Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									142,893	127,075
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition									0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition									0	0
Revenue									142,893	127,075
Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									644,309	728,746
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition									7,989	(1,304)
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition									0	0
Revenue									652,298	727,442
Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									1,610,568	1,779,116
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition									7,989	(1,304)
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition									93,691	(2,272)
Revenue									1,712,248	1,775,540
All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									100,901	147,051
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition									0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition									(10,630)	2,658
Revenue									90,271	149,709
Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									(256,228)	(231,917)
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition									0	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition									0	0
Revenue									(256,228)	(231,917)
Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									1,455,241	1,694,250
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition									7,989	(1,304)
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition									83,061	386
Revenue									1,546,291	1,693,332
Production of Natural Gas [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									402,447	481,048
Production of Natural Gas [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Natural Gas [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Natural Gas [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Natural Gas [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									402,447	481,048
Production of Natural Gas [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Natural Gas [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Natural Gas [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									402,447	481,048
Production of Crude Oil [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									107,844	149,078
Production of Crude Oil [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Crude Oil [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Crude Oil [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Crude Oil [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									107,844	149,078
Production of Crude Oil [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Crude Oil [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Production of Crude Oil [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									107,844	149,078
Natural Gas Processing [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									2,374	3,277
Natural Gas Processing [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Processing [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Processing [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Processing [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									2,374	3,277
Natural Gas Processing [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Processing [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Processing [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									2,374	3,277
Natural Gas Gathering Service [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Gathering Service [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Gathering Service [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									142,893	127,064
Natural Gas Gathering Service [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Gathering Service [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									142,893	127,064
Natural Gas Gathering Service [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Gathering Service [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									(142,821)	(127,064)
Natural Gas Gathering Service [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									72	0
Natural Gas Transportation Service [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Transportation Service [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									229,391	209,184
Natural Gas Transportation Service [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Transportation Service [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									109,214	119,253
Natural Gas Transportation Service [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									338,605	328,437
Natural Gas Transportation Service [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Transportation Service [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									(77,699)	(70,689)
Natural Gas Transportation Service [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									260,906	257,748
Natural Gas Storage Service [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Storage Service [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									79,073	75,484
Natural Gas Storage Service [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Storage Service [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Storage Service [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									79,073	75,484
Natural Gas Storage Service [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Storage Service [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									(34,579)	(32,488)
Natural Gas Storage Service [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									44,494	42,996
Natural Gas Residential Sales [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Residential Sales [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Residential Sales [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Residential Sales [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									475,846	539,962
Natural Gas Residential Sales [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									475,846	539,962
Natural Gas Residential Sales [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Residential Sales [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Residential Sales [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									475,846	539,962
Natural Gas Commercial Sales [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Commercial Sales [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Commercial Sales [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Commercial Sales [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									61,239	73,331
Natural Gas Commercial Sales [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									61,239	73,331
Natural Gas Commercial Sales [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Commercial Sales [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Commercial Sales [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									61,239	73,331
Natural Gas Industrial Sales [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Industrial Sales [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Industrial Sales [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Industrial Sales [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									3,291	4,830
Natural Gas Industrial Sales [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									3,291	4,830
Natural Gas Industrial Sales [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Industrial Sales [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Industrial Sales [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									3,291	4,830
Natural Gas Marketing [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Marketing [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Marketing [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Marketing [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Marketing [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	0
Natural Gas Marketing [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									95,727	143,627
Natural Gas Marketing [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									(835)	(1,127)
Natural Gas Marketing [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									94,892	142,500
Other [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									1,097	1,609
Other [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									1,140	3,615
Other [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									0	11
Other [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									(5,281)	(8,630)
Other [Member] | Total Reportable Segments [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									(3,044)	(3,395)
Other [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									5,174	3,424
Other [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									(294)	(549)
Other [Member] | Total Consolidated [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers									$ 1,836	$ (520)