Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Deferred tax assets:
Employee benefit plans	$ 70,929	$ 72,745
Interest rate swaps	33,918	27,135
Net operating loss carryforwards	485,133	461,481
Charitable and other credit carryforwards	8,241	6,818
Regulatory excess deferred tax	165,701	169,947
Other	13,186	13,804
Total deferred tax assets	777,108	751,930
Valuation allowance	(1,894)	(1,465)
Net deferred tax assets	775,214	750,465
Deferred tax liabilities:
Difference in net book value and net tax value of assets	(2,004,516)	(1,859,787)
Pension funding	(4,384)	(6,986)
Gas cost adjustments	(18,072)	1,005
Other	(48,257)	(38,764)
Total deferred tax liabilities	(2,075,229)	(1,904,532)
Net deferred tax liabilities	(1,300,015)	(1,154,067)
Deferred credits for rate regulated entities	$ 2,582	$ 762