Asset Acquisitions and Divestitures (Narrative) (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2020 USD ($) a Bcf	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Aug. 05, 2020 USD ($)
Asset Acquisition and Divestitures [Line Items]
Total Consideration for Asset Acquisition	$ 506,258
Net Acres Acquired in Appalachia | a	400,000
Net Acres Acquired in Tioga County | a	200,000
Proved Developed Reserves (Volume) | Bcf	684,141
Anticipated Proceeds from Sale of Certain Timber and Other Assets				$ 115,700
Assets Held for Sale, Net	$ 53,424	$ 0
Net Proceeds from Sale of Oil and Gas Producing Properties	$ 0	$ 0	$ 55,506
Seneca [Member]
Asset Acquisition and Divestitures [Line Items]
Net Proceeds from Sale of Oil and Gas Producing Properties			17,300
Sespe Field [Member]
Asset Acquisition and Divestitures [Line Items]
Net Proceeds from Sale of Oil and Gas Producing Properties			$ 38,200