Income Taxes - Provisions for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Current expense (benefit):
Federal	$ (26,952)	$ (12,253)	$ (17,845)
State	10,729	5,739	6,805
Total current (benefit) expense	(16,223)	(6,514)	(11,040)
Deferred expense (benefit):
Federal	60,766	70,293	71,005
State	1,486	8,593	6,262
Investment tax credit amortization	(318)	(318)	(329)
Total deferred expense	61,934	78,568	76,938
Total income tax expense	$ 45,711	$ 72,054	$ 65,898