Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Deferred tax assets:
Employee benefit plans	$ 66,991	$ 70,929
Interest rate swaps	16,719	33,918
Net operating loss carryforwards	476,507	485,133
Charitable and other credit carryforwards	8,712	8,241
Regulatory excess deferred tax	161,565	165,701
Other	73,542	13,186
Total deferred tax assets	804,036	777,108
Valuation allowance	(1,102)	(1,894)
Net deferred tax assets	802,934	775,214
Deferred tax liabilities:
Difference in net book value and net tax value of assets	(2,138,966)	(2,004,516)
Pension funding	(484)	(4,384)
Gas cost adjustments	(23,209)	(18,072)
Other	(96,844)	(48,257)
Total deferred tax liabilities	(2,259,503)	(2,075,229)
Net deferred tax liabilities	(1,456,569)	(1,300,015)
Deferred credits for rate regulated entities	2,537	2,582
Deferred tax liabilities related to goodwill	$ 129,000	$ 131,000