Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at Beginning of Year	$ 156,805	$ 117,713	$ 119,511
Liabilities Incurred	2,719	4,433	5,390
Revisions of Estimates	16,721	33,717	(7,886)
Liabilities Settled	(72,215)	(6,825)	(6,955)
Accretion Expense	8,300	7,767	7,653
Balance at End of Year	$ 112,330	$ 156,805	$ 117,713