Income Taxes - Provisions for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Current expense (benefit):
Federal	$ (17,845)	$ 34,990	$ 38,786
State	6,805	15,138	11,449
Total current (benefit) expense	(11,040)	50,128	50,235
Deferred expense (benefit):
Federal	71,005	28,877	29,208
State	6,262	815	4,717
Investment tax credit amortization	(329)	(336)	(337)
Total income tax expense	$ 65,898	$ 79,484	$ 83,823