Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Current income taxes
Federal	$ 0	$ 0	$ 0
State	7,251	5,527	8,178
Deferred income taxes
Federal	175,897	169,106	124,836
State	12,548	12,375	9,605
Investment tax credits	(6)	(6)	(20)
Income tax expense	195,690	187,002	142,599
Deferred tax assets
Employee benefit plans	121,619	116,157
Interest rate agreements	51,067	10,565
Tax net operating loss carryforwards	313,224	236,626
Tax credit carryforward	22,281	21,614
Other	36,695	28,849
Total deferred tax assets	544,886	413,811
Valuation allowance	(10,872)	(6,969)
Net deferred tax assets	534,014	406,842
Deferred tax liabilities
Difference in net book value and net tax value of assets	(1,890,886)	(1,655,894)
Pension funding	(35,247)	(17,890)
Gas Cost Adjustments	(43,634)	(31,252)
Other	(31,480)	(37,173)
Total deferred tax liabilities	(2,001,247)	(1,742,209)
Net deferred tax liabilities	(1,467,233)	(1,335,367)
Deferred credits for rate regulated entities	412	(109)
Deferred Tax Assets, Charitable Contribution Carryforwards [Abstract]
Deferred Tax Assets, Charitable Contribution Carryforwards	$ 11,400
Charitable Contribution Carryforwards, Expiration Date, Minimum	Dec. 31, 2015
Charitable Contribution Carryforwards, Expiration Date, Maximum	Dec. 31, 2020
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	$ 12,629	4,158
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions	1,009	3,846
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	3,431	4,625
Unrecognized Tax Benefits, Ending Balance	17,069	12,629	$ 4,158
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	820	411
Unrecognized Tax Benefits, Gross	17,889	13,040
Unrecognized Tax Benefits, Deferred Federal and State Income Tax Benefits	(6,261)	(4,564)
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	11,628	8,476
Valuation Allowance, Tax Credit Carryforward [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Charged to Cost and Expense	4,400	$ 7,000
Valuation Allowances and Reserves, Deductions	1,100
Valuation Allowance, Operating Loss Carryforwards [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Charged to Cost and Expense	600
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	14,900
Operating Loss Carryforwards, Federal Tax Effects	$ 8,000
State and Local Jurisdiction [Member] | Minimum [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2016
State and Local Jurisdiction [Member] | Maximum [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2030
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	$ 307,300
Domestic Tax Authority [Member] | Minimum [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2029