Retirement Plan And Other Post-Retirement Benefits (Narrative) (Details) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Increase to accumulated other comprehensive income	$ 44,731,000	$ 171,586,000	$ (169,919,000)
Expected future benefit payments in year one	64,400,000
Expected future benefit payments in year two	65,000,000
Expected future benefit payments in year three	65,400,000
Expected future benefit payments in year four	65,800,000
Expected future benefit payments in year five	66,200,000
Expected future benefit payments in five years thereafter	331,100,000
Effect of one percentage point increase on accumulated postretirement benefit obligation	57,900,000
Effect of one percentage point increase on service and interest cost components	3,300,000
Effect of one percentage point decrease on accumulated postretirement benefit obligation	48,500,000
Effect of one percentage point decrease on service and interest cost components	2,700,000
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	7,500,000	7,000,000
Accumulated benefit obligation	72,500,000	72,400,000	66,000,000
Benefit obligation	$ 88,900,000	$ 91,700,000	$ 85,800,000
Discount rate	3.22%	2.80%	3.50%
Rate of compensation increase	7.75%	7.75%	7.75%
Tax-Deferred Savings Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Costs Recognized	$ 5,900,000	$ 5,900,000	$ 5,800,000
Retirement Savings Account [Member]
Defined Benefit Plan Disclosure [Line Items]
Costs Recognized	2,900,000	2,600,000	2,300,000
Other Than Veba Trust And 401(h) Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions	100,000
Non-Qualified Benefit Plans, Other Post-Retirement Benefit Plan And Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in other regulatory assets	163,300,000
Increase to accumulated other comprehensive income	29,100,000
Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	34,848,000	32,095,000	37,700,000
Accumulated benefit obligation	1,010,179,000	1,039,408,000	968,984,000
Benefit obligation	$ 1,054,826,000	$ 1,097,421,000	$ 1,026,190,000	$ 999,499,000
Discount rate	3.77%	3.60%	4.25%
Rate of compensation increase	4.70%	4.70%	4.75%
Employer Contributions	$ 17,146,000	$ 7,000,000	$ 36,200,000
Expected long term rate of return on plan assets	7.00%	7.25%	7.50%
Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 14,092,000	$ 12,923,000	$ 10,446,000
Benefit obligation	$ 462,619,000	$ 526,138,000	$ 464,987,000	$ 465,583,000
Discount rate	3.81%	3.70%	4.50%
Rate of compensation increase	4.70%	4.70%	4.75%
Employer Contributions	$ 3,853,000	$ 2,839,000	$ 2,161,000
Expected future benefit payments in year one	26,483,000
Expected future benefit payments in year two	27,456,000
Expected future benefit payments in year three	28,359,000
Expected future benefit payments in year four	29,173,000
Expected future benefit payments in year five	29,757,000
Expected future benefit payments in five years thereafter	$ 152,957,000
Expected long term rate of return on plan assets	6.50%	6.75%	7.00%
VEBA Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions	$ 3,800,000
Other Actuarial Experience [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(3,600,000)
Other Actuarial Experience [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(50,300,000)	$ 11,000,000	$ 12,800,000
Mortality Assumption Change [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation			24,200,000
Mortality Improvement Projection Scale [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(8,300,000)
Mortality Improvement Projection Scale [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(5,700,000)
Discount Rate Change [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	(20,500,000)	78,500,000
Discount Rate Change [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in benefit obligation	$ (6,200,000)	49,400,000	(14,300,000)
Effective Fiscal 2018 [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets	7.00%
Effective Fiscal 2018 [Member] | Other Post-Retirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long term rate of return on plan assets	6.25%
Minimum [Member] | Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 15,000,000
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	40.00%
Minimum [Member] | Fixed Income Securities [Member] | Retirement Plan and Veba Trusts And 401H Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	40.00%
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	$ 40,000,000
Maximum [Member] | VEBA Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions in next fiscal year	$ 4,000,000
Maximum [Member] | Equity Securities [Member] | Retirement Plan and Veba Trusts And 401H Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	60.00%
Maximum [Member] | Fixed Income Securities [Member] | Retirement Plan and Veba Trusts And 401H Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	60.00%
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	$ 14,100,000	9,800,000	4,500,000
Non-Current [Member] | Non-Qualified Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ 74,800,000	$ 81,900,000	$ 81,300,000