Summary Of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2019	Sep. 30, 2019
Summary Of Significant Accounting Policies [Line Items]
Gas Stored Underground	$ 5,745			$ 5,745		$ 33,338
Capitalized Costs Oil and Gas Producing Activities Net	$ 1,800,000			1,800,000		1,800,000
Capitalized costs of unproved properties excluded from amortization				$ 149,200		148,100
Full cost ceiling test discount factor	10.00%			10.00%
Impairment of Oil and Gas Producing Properties	$ 0	$ 76,200	$ 177,761	$ 76,152	$ 177,761
Increase estimated future net cash flows	$ 127,900
Antidilutive securities	334,945		310,015	334,945	406,748
Retained Earnings (Accumulated Deficit)	$ 1,100,718	1,028,844	$ 1,176,870	$ 1,100,718	$ 1,176,870	991,630		$ 1,320,592	$ 1,272,601
Amount Full Cost Ceiling Exceeds Book Value Of Oil And Gas Properties	145,400			145,400
Earnings Reinvested in The Business [Member]
Summary Of Significant Accounting Policies [Line Items]
Retained Earnings (Accumulated Deficit)	1,100,718	1,028,844	$ 1,176,870	1,100,718	$ 1,176,870	991,630		1,320,592	1,272,601
Reserve For Gas Replacement [Member]
Summary Of Significant Accounting Policies [Line Items]
Gas Stored Underground	$ 21,432			$ 21,432		0
Impairment of Oil and Gas Producing Properties [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred Income Tax Benefit		21,000
Restricted Stock Units [Member]
Summary Of Significant Accounting Policies [Line Items]
Share based compensation other than options grants in period				172,513
Granted in fiscal year, weighted average grant date fair value				$ 37.98
Performance Shares [Member]
Summary Of Significant Accounting Policies [Line Items]
Share based compensation other than options grants in period				309,470
Granted in fiscal year, weighted average grant date fair value				$ 39.19
Subsequent Event [Member] | Reserve For Gas Replacement [Member]
Summary Of Significant Accounting Policies [Line Items]
Gas Stored Underground							$ 0
Guidance for Hedging [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member]
Summary Of Significant Accounting Policies [Line Items]
Retained Earnings (Accumulated Deficit)		$ 0				$ 0		$ 0	(950)
Increase (Decrease) to Accumulated Other Comprehensive Income									1,000
Guidance for Hedging [Member] | Cumulative Effect of Adoption of Authoritative Guidance [Member] | Earnings Reinvested in The Business [Member]
Summary Of Significant Accounting Policies [Line Items]
Retained Earnings (Accumulated Deficit)									$ (950)