Asset Acquisition and Divestitures (Summary of Asset Acquisition) (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Asset Acquisition and Divestitures [Line Items]
Purchase Price		$ 503,908
Transaction Costs		2,350
Total Consideration		506,258
Property, Plant and Equipment		1,228,617	$ 781,246	$ 600,602
Inventory		1,241
Total Accounting		506,258
Shell Acquisition [Member]
Asset Acquisition and Divestitures [Line Items]
Property, Plant and Equipment		505,017
Exploration And Production [Member]
Asset Acquisition and Divestitures [Line Items]
Property, Plant and Equipment		670,455	491,889	380,677
Inventory		1,132
Total Accounting		282,780
Proved Developed Producing Properties		241,134
Property Plant and Equipment Capitalized in the Full Cost Pool		277,832
Exploration And Production [Member] | Shell Acquisition [Member]
Asset Acquisition and Divestitures [Line Items]
Property, Plant and Equipment	[1],[2]	281,648
Gathering [Member]
Asset Acquisition and Divestitures [Line Items]
Property, Plant and Equipment		297,806	$ 49,650	$ 61,728
Inventory		109
Total Accounting		223,478
Gathering [Member] | Shell Acquisition [Member]
Asset Acquisition and Divestitures [Line Items]
Property, Plant and Equipment	[2]	$ 223,369

[1] Includes $241,134 in Proved Developed Producing Properties and $277,832 capitalized in the full cost pool.

[2] The Company utilized an income approach and market based approach to determine the fair value of the acquired property, plant and equipment in the Exploration and Production reporting segment. The Company utilized a cost approach and an income approach to determine the fair value of the acquired property, plant and equipment in the Gathering reporting segment.