Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Current income taxes
Federal	$ 0	$ 0	$ 0
State	9,022	5,667	6,513
Deferred income taxes
Federal	197,013	178,630	170,649
State	15,348	12,350	12,393
Investment tax credits	0	(5)	(6)
Income tax expense	221,383	196,642	189,549
Deferred tax assets
Employee benefit plans	121,288	122,682
Interest rate agreements	65,171	107,782
Tax net operating loss carryforwards	555,043	514,391
Charitable and other credit carryforwards	18,873	22,273
Other	10,218	23,648
Total deferred tax assets	770,593	790,776
Valuation allowance	(5,403)	(10,481)
Net deferred tax assets	765,190	780,295
Deferred tax liabilities
Difference in net book value and net tax value of assets	(2,528,485)	(2,259,278)
Pension funding	(13,101)	(30,652)
Gas Cost Adjustments	(60,376)	(54,725)
Other	(41,927)	(38,696)
Total deferred tax liabilities	(2,643,889)	(2,383,351)
Net deferred tax liabilities	(1,878,699)	(1,603,056)
Deferred credits for rate regulated entities	985	861
Deferred Tax Assets, Charitable Contribution Carryforwards [Abstract]
Deferred Tax Assets, Charitable Contribution Carryforwards	$ 7,600
Charitable Contribution Carryforwards, Expiration Date, Minimum	Dec. 31, 2018
Charitable Contribution Carryforwards, Expiration Date, Maximum	Dec. 31, 2022
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	$ 20,298	17,069	12,629
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions			1,009
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions	(366)	(290)
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	3,787	3,519	3,431
Unrecognized Tax Benefits, Ending Balance	23,719	20,298	17,069
Unrecognized Tax Benefits, Deferred Federal and State Income Tax Benefits	(8,302)	(7,104)	(5,974)
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	15,417	13,194	11,095
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	1,100	2,500	500
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	4,500	3,300	$ 800
Valuation Allowance, Tax Credit Carryforward [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Charged to Cost and Expense	0	$ 1,100
Valuation Allowances and Reserves, Deductions	5,100
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	22,100
Operating Loss Carryforwards, Federal Tax Effects	$ 11,900
State and Local Jurisdiction [Member] | Minimum [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2018
State and Local Jurisdiction [Member] | Maximum [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2032
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	$ 532,900
Domestic Tax Authority [Member] | Minimum [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2029