Income Taxes - Provisions for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Current expense:
Federal	$ 34,990	$ 38,786	$ 21,807
State	15,138	11,449	11,824
Deferred expense (benefit):	50,128	50,235	33,631
Deferred expense (benefit):
Federal	28,877	29,208	33,349
State	815	4,717	2,274
Investment tax credit amortization	(336)	(337)	(342)
Total income tax expense	$ 79,484	$ 83,823	$ 68,912