Retirement Plan And Other Post-Retirement Benefits (Narrative) (Details) - USD ($)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Increase to accumulated other comprehensive income	$ 547,569,000	$ 85,973,000	$ (36,270,000)
Level 1 or Level 2 Transfers	0	0
Benefit assets transferred in/out of Level 3	0	0
Non-Qualified Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	8,300,000	8,900,000	7,600,000
Accumulated benefit obligation	76,900,000	78,700,000	79,800,000
Benefit obligation	$ 95,800,000	$ 98,100,000	$ 99,500,000
Discount rate	2.15%	1.92%	2.77%
Rate of compensation increase	8.00%	8.00%	8.00%
Tax-Deferred Savings Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Costs Recognized	$ 7,200,000	$ 6,700,000	$ 6,400,000
Retirement Savings Account [Member]
Defined Benefit Plan Disclosure [Line Items]
Costs Recognized	4,800,000	4,200,000	3,900,000
Other Than Veba Trust And 401(h) Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions	300,000
Non-Qualified Benefit Plans, Other Post-Retirement Benefit Plan And Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in other regulatory assets	130,900,000
Increase to accumulated other comprehensive income	34,100,000
Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	24,911,000	24,657,000	19,907,000
Accumulated benefit obligation	1,060,659,000	1,096,427,000	1,053,914,000
Benefit obligation	$ 1,098,456,000	$ 1,139,105,000	$ 1,097,625,000	$ 985,690,000
Discount rate	2.75%	2.66%	3.15%
Rate of compensation increase	4.70%	4.70%	4.70%
Employer Contributions	$ 20,000,000	$ 24,621,000	$ 29,215,000
Expected future benefit payments in year one	67,000,000
Expected future benefit payments in year two	67,100,000
Expected future benefit payments in year three	67,200,000
Expected future benefit payments in year four	66,900,000
Expected future benefit payments in year five	66,400,000
Expected future benefit payments in five years thereafter	$ 320,200,000
Expected long term rate of return on plan assets	6.00%	6.40%	6.75%
Retirement Plan [Member] | Discount Rate Change [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ (11,200,000)	$ 61,300,000	$ 128,400,000
Retirement Plan [Member] | Other Actuarial Experience [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	5,900,000
Retirement Plan [Member] | Mortality Improvement Projection Scale [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(2,900,000)
Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	10,371,000	10,992,000	10,521,000
Benefit obligation	$ 431,213,000	$ 476,722,000	$ 468,163,000	$ 435,986,000
Discount rate	2.76%	2.71%	3.17%
Rate of compensation increase	4.70%	4.70%	4.70%
Employer Contributions	$ 3,068,000	$ 3,080,000	$ 3,064,000
Expected future benefit payments in year one	27,230,000
Expected future benefit payments in year two	27,442,000
Expected future benefit payments in year three	27,601,000
Expected future benefit payments in year four	27,691,000
Expected future benefit payments in year five	27,643,000
Expected future benefit payments in five years thereafter	$ 134,674,000
Expected long term rate of return on plan assets	5.40%	5.70%	6.00%
Other Post-Retirement Benefit Plans [Member] | Discount Rate Change [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ (2,500,000)	$ 25,400,000	$ 57,200,000
Other Post-Retirement Benefit Plans [Member] | Other Actuarial Experience [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(26,600,000)	(6,500,000)	(18,900,000)
Other Post-Retirement Benefit Plans [Member] | Mortality Improvement Projection Scale [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(2,000,000)	(2,500,000)	(3,900,000)
Other Post-Retirement Benefit Plans [Member] | Health Care Cost Trend Rates [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(3,700,000)
VEBA Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions	$ 2,800,000
Effective Fiscal 2022 [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets	5.20%
Effective Fiscal 2022 [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets	5.20%
Minimum [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 20,000,000
Minimum [Member] | VEBA Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	2,500,000
Maximum [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	25,000,000
Maximum [Member] | VEBA Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	3,000,000
Other Accruals And Current Liabilities [Member] | Non-Qualified Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	15,408,000	14,460,000	13,200,000
Non-Current [Member] | Non-Qualified Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ 80,400,000	$ 83,600,000	$ 86,300,000