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Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Sep. 30, 2024
Deferred tax assets:    
Employee benefit plans $ 34,202 $ 41,184
Net operating loss carryforwards 473,642 484,816
Charitable and other credit carryforwards 14,200 12,301
Regulatory excess deferred tax 31,387 46,330
Lease asset 72,973 63,747
Other 31,145 34,934
Total deferred tax assets 657,549 683,312
Valuation allowance (1,365) (1,457)
Net deferred tax assets 656,184 681,855
Deferred tax liabilities:    
Difference in net book value and net tax value of assets (3,200,535) (2,914,854)
Gas cost adjustments (43,819) (49,443)
Winter Storm Uri regulatory asset (17,993) (20,846)
Lease liability (66,831) (57,177)
Rate deferral adjustment (64,640) (50,571)
Interest rate agreements (135,640) (134,536)
Other (45,073) (47,770)
Total deferred tax liabilities (3,574,531) (3,275,197)
Net deferred tax liabilities $ (2,918,347) $ (2,593,342)