Retirement Plan And Other Post-Retirement Benefits (Narrative) (Details) - USD ($)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Decrease to accumulated other comprehensive income	$ (36,270,000)	$ 57,283,000	$ 44,731,000
Effect of one percentage point increase on accumulated postretirement benefit obligation	60,800,000
Effect of one percentage point increase on service and interest cost components	2,700,000
Effect of one percentage point decrease on accumulated postretirement benefit obligation	49,100,000
Effect of one percentage point decrease on service and interest cost components	2,100,000
Benefit assets transferred	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	7,600,000	6,800,000	7,600,000
Accumulated benefit obligation	79,800,000	70,600,000	72,500,000
Benefit obligation	$ 99,500,000	$ 86,100,000	$ 88,900,000
Discount rate	2.77%	4.02%	3.22%
Rate of compensation increase	8.00%	7.75%	7.75%
Tax-Deferred Savings Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Costs Recognized	$ 6,400,000	$ 6,200,000	$ 5,900,000
Retirement Savings Account [Member]
Defined Benefit Plan Disclosure [Line Items]
Costs Recognized	3,900,000	3,500,000	2,900,000
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]
Increase in other regulatory assets	82,700,000
Decrease to accumulated other comprehensive income	36,800,000
Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	19,907,000	28,382,000	34,848,000
Accumulated benefit obligation	1,053,914,000	946,763,000	1,010,179,000
Benefit obligation	$ 1,097,625,000	$ 985,690,000	$ 1,054,826,000	$ 1,097,421,000
Discount rate	3.15%	4.30%	3.77%
Rate of compensation increase	4.70%	4.70%	4.70%
Employer Contributions	$ 29,215,000	$ 32,980,000	$ 17,146,000
Expected future benefit payments in year one	66,300,000
Expected future benefit payments in year two	66,800,000
Expected future benefit payments in year three	67,100,000
Expected future benefit payments in year four	67,100,000
Expected future benefit payments in year five	67,100,000
Expected future benefit payments in five years thereafter	$ 328,700,000
Expected long term rate of return on plan assets	6.75%	7.00%	7.00%
Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 10,521,000	$ 10,306,000	$ 14,092,000
Benefit obligation	$ 468,163,000	$ 435,986,000	$ 462,619,000	$ 526,138,000
Discount rate	3.17%	4.31%	3.81%
Rate of compensation increase	4.70%	4.70%	4.70%
Employer Contributions	$ 3,064,000	$ 2,896,000	$ 3,853,000
Expected future benefit payments in year one	27,998,000
Expected future benefit payments in year two	28,711,000
Expected future benefit payments in year three	29,142,000
Expected future benefit payments in year four	29,478,000
Expected future benefit payments in year five	29,631,000
Expected future benefit payments in five years thereafter	$ 147,138,000
Expected long term rate of return on plan assets	6.00%	6.25%	6.50%
VEBA Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions	$ 2,800,000
Other Actuarial Experience [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	4,700,000
Other Actuarial Experience [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(18,900,000)	$ 7,300,000	$ (50,300,000)
Mortality Improvement Projection Scale [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(5,300,000)
Mortality Improvement Projection Scale [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(3,900,000)	(2,400,000)	(5,700,000)
Discount Rate Change [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	128,400,000	(58,100,000)	(20,500,000)
Discount Rate Change [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ 57,200,000	(25,800,000)	(6,200,000)
Effective Fiscal 2020 [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets	6.40%
Effective Fiscal 2020 [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets	5.70%
Minimum [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 25,000,000.0
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	$ 30,000,000.0
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	$ 13,194,000	11,536,000	14,100,000
Non-Current [Member] | Non-Qualified Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ 86,300,000	$ 74,600,000	$ 74,800,000