Income Taxes (Schedule Of Income Tax Reconciliation By Applying Federal Income Tax Rate) (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended	12 Months Ended
		Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income Taxes [Line Items]
U.S. Income (Loss) Before Income Taxes				$ (105,046)	$ 389,420	$ 383,922
Income Tax Expense, Computed at U.S. Federal Statutory Rate	[1]			(22,060)	81,778	94,061
State Valuation Allowance	[2]			63,205	0	0
State Income Taxes (Benefit)	[3]			(18,374)	22,397	22,203
Federal Tax Credits				(217)	(7,361)	(6,576)
Amortization of Excess Deferred Federal Income Taxes	[4]			(4,749)	(3,185)	(1,336)
Plant Flow Through Items				(2,848)	(1,544)	(872)
Stock Compensation				3,867	(1,491)	(321)
Impact of 2017 Tax Reform Act	[5]			0	(5,000)	(112,598)
Miscellaneous				(98)	(464)	(2,160)
Total Income Taxes				$ 18,726	$ 85,130	$ (7,599)
Federal Statutory Rate		35.00%	21.00%	21.00%	21.00%	24.50%	35.00%
Reversal of estimate for potential sequestration of AMT credit refunds					$ 5,000
Estimate for potential sequestration of AMT credit refunds						$ 5,000
Income tax benefit from blended tax rate						$ 9,100

[1] For fiscal 2020 and 2019, the statutory rate of 21% was utilized. For fiscal 2018, a blended rate of 24.5% was utilized, calculated as 35% for the first quarter of the fiscal year and 21% for the remaining three quarters.

[2] During fiscal 2020, a valuation allowance was recorded against certain state deferred tax assets, as discussed below.

[3] The state income tax expense (benefit) shown above includes the impact of state enhanced oil recovery tax credits and adjustments to the estimated state effective tax rates utilized in the calculation of deferred income taxes.

[4] Represents amortization of net excess deferred federal income taxes under the 2017 Tax Reform Act.

[5] The $5.0 million benefit in fiscal 2019 represents the reversal of the estimated sequestration of AMT credit refunds. The amount for fiscal 2018 represents the remeasurement of deferred income taxes as a result of the lower U.S. corporate income tax rate, including a $5.0 million estimate for the potential sequestration of AMT credit refunds and the benefit of $9.1 million as a result of the blended tax rate.