Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Deferred tax assets:
Employee benefit plans	$ 64,316	$ 66,991
Interest rate swaps	0	16,719
Net operating loss carryforwards	911,424	476,507
Charitable and other credit carryforwards	7,712	8,712
Regulatory excess deferred tax	148,200	161,565
Lease asset	52,138	53,118
Other	33,591	20,424
Total deferred tax assets	1,217,381	804,036
Valuation allowance	(663)	(1,102)
Net deferred tax assets	1,216,718	802,934
Deferred tax liabilities:
Difference in net book value and net tax value of assets	(2,258,264)	(2,138,966)
Gas cost adjustments	(26,413)	(23,209)
Winter Storm Uri regulatory asset	(471,025)	0
Lease liability	(52,138)	(53,118)
Rate deferral adjustment	(47,445)	(21,945)
Interest rate agreements	(20,156)	0
Other	(47,086)	(22,265)
Total deferred tax liabilities	(2,922,527)	(2,259,503)
Net deferred tax liabilities	(1,705,809)	(1,456,569)
Deferred credits for rate regulated entities	$ 4,181	2,537
Deferred tax liabilities related to goodwill		$ 129,000