Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		$ (625,733)	$ (513,597)
Other Comprehensive Gains and Losses Before Reclassifications		486,560	(755,831)
Amounts Reclassified From Other Comprehensive Loss		84,113	649,502
Other Post-Retirement Adjustment for Regulatory Proceeding	$ (5,800)		(5,807)
Ending balance		(55,060)	(625,733)
Gains and Losses on Derivative Financial Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(572,163)	(449,962)
Other Comprehensive Gains and Losses Before Reclassifications		493,936	(763,223)
Amounts Reclassified From Other Comprehensive Loss		82,850	641,022
Other Post-Retirement Adjustment for Regulatory Proceeding			0
Ending balance		4,623	(572,163)
Funded Status of the Pension and Other Post-Retirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(53,570)	(63,635)
Other Comprehensive Gains and Losses Before Reclassifications		(7,376)	7,392
Amounts Reclassified From Other Comprehensive Loss		1,263	8,480
Other Post-Retirement Adjustment for Regulatory Proceeding			(5,807)
Ending balance		$ (59,683)	$ (53,570)