Retirement Plan And Other Post-Retirement Benefits (Narrative) (Details) - USD ($)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Decrease to accumulated other comprehensive income	$ 85,973,000	$ (36,270,000)	$ 57,283,000
Effect of one percentage point increase on accumulated postretirement benefit obligation	61,700,000
Effect of one percentage point increase on service and interest cost components	2,500,000
Effect of one percentage point decrease on accumulated postretirement benefit obligation	51,500,000
Effect of one percentage point decrease on service and interest cost components	1,800,000
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	8,900,000	7,600,000	6,800,000
Accumulated benefit obligation	78,700,000	79,800,000	70,600,000
Benefit obligation	$ 98,100,000	$ 99,500,000	$ 86,100,000
Discount rate	1.92%	2.77%	4.02%
Rate of compensation increase	8.00%	8.00%	7.75%
Tax-Deferred Savings Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Costs Recognized	$ 6,700,000	$ 6,400,000	$ 6,200,000
Retirement Savings Account [Member]
Defined Benefit Plan Disclosure [Line Items]
Costs Recognized	4,200,000	3,900,000	3,500,000
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	3,800,000
Decrease to accumulated other comprehensive income	3,900,000
Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	24,657,000	19,907,000	28,382,000
Accumulated benefit obligation	1,096,427,000	1,053,914,000	946,763,000
Benefit obligation	$ 1,139,105,000	$ 1,097,625,000	$ 985,690,000	$ 1,054,826,000
Discount rate	2.66%	3.15%	4.30%
Rate of compensation increase	4.70%	4.70%	4.70%
Employer Contributions	$ 24,621,000	$ 29,215,000	$ 32,980,000
Expected future benefit payments in year one	66,800,000
Expected future benefit payments in year two	67,200,000
Expected future benefit payments in year three	67,400,000
Expected future benefit payments in year four	67,400,000
Expected future benefit payments in year five	67,100,000
Expected future benefit payments in five years thereafter	$ 326,800,000
Expected long term rate of return on plan assets	6.40%	6.75%	7.00%
Retirement Plan [Member] | Discount Rate Change [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ 61,300,000	$ 128,400,000	$ (58,100,000)
Retirement Plan [Member] | Other Actuarial Experience [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	7,900,000
Retirement Plan [Member] | Mortality Improvement Projection Scale [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(3,300,000)
Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	10,992,000	10,521,000	10,306,000
Benefit obligation	$ 476,722,000	$ 468,163,000	$ 435,986,000	$ 462,619,000
Discount rate	2.71%	3.17%	4.31%
Rate of compensation increase	4.70%	4.70%	4.70%
Employer Contributions	$ 3,080,000	$ 3,064,000	$ 2,896,000
Expected future benefit payments in year one	28,361,000
Expected future benefit payments in year two	28,820,000
Expected future benefit payments in year three	29,085,000
Expected future benefit payments in year four	29,192,000
Expected future benefit payments in year five	29,231,000
Expected future benefit payments in five years thereafter	$ 143,696,000
Expected long term rate of return on plan assets	5.70%	6.00%	6.25%
Other Post-Retirement Benefit Plans [Member] | Discount Rate Change [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ 25,400,000	$ 57,200,000	$ (25,800,000)
Other Post-Retirement Benefit Plans [Member] | Other Actuarial Experience [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(6,500,000)	(18,900,000)	7,300,000
Other Post-Retirement Benefit Plans [Member] | Mortality Improvement Projection Scale [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(2,500,000)	(3,900,000)	(2,400,000)
VEBA Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions	$ 2,800,000
Effective Fiscal 2021 [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets	6.00%
Effective Fiscal 2021 [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets	5.40%
Minimum [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 15,000,000.0
Minimum [Member] | VEBA Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 2,500,000
Minimum [Member] | Equity Securities [Member] | Retirement Plan and Veba Trusts And 401H Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	30.00%
Minimum [Member] | Fixed Income Securities [Member] | Retirement Plan and Veba Trusts And 401H Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	50.00%
Minimum [Member] | Other Securities [Member] | Retirement Plan and Veba Trusts And 401H Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	0.00%
Maximum [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 25,000,000.0
Maximum [Member] | VEBA Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 3,000,000.0
Maximum [Member] | Equity Securities [Member] | Retirement Plan and Veba Trusts And 401H Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	50.00%
Maximum [Member] | Fixed Income Securities [Member] | Retirement Plan and Veba Trusts And 401H Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	70.00%
Maximum [Member] | Other Securities [Member] | Retirement Plan and Veba Trusts And 401H Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	15.00%
Other Accruals And Current Liabilities [Member] | Non-Qualified Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ 14,460,000	13,194,000	11,500,000
Non-Current [Member] | Non-Qualified Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ 83,600,000	$ 86,300,000	$ 74,600,000