Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Deferred tax assets:
Employee benefit plans	$ 72,745	$ 121,288
Interest rate agreements	27,135	65,171
Net operating loss carryforwards	461,481	555,043
Charitable and other credit carryforwards	6,818	18,873
Regulatory excess deferred tax	169,947	0
Other	13,804	10,218
Total deferred tax assets	751,930	770,593
Valuation allowance	(1,465)	(5,403)
Net deferred tax assets	750,465	765,190
Deferred tax liabilities:
Difference in net book value and net tax value of assets	(1,859,787)	(2,528,485)
Pension funding	(6,986)	(13,101)
Gas cost adjustments	1,005	(60,376)
Other	(38,764)	(41,927)
Total deferred tax liabilities	(1,904,532)	(2,643,889)
Net deferred tax liabilities	(1,154,067)	(1,878,699)
Deferred credits for rate regulated entities	$ 762	$ 985