Quarterly Data (unaudited) - Schedule of Quarterly Financial Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Quarterly Financial Data [Abstract]
Revenues	$ 121,340	$ 152,694	$ 513,956	$ 298,899	$ 122,216	$ 148,798	$ 395,901	$ 273,797	$ 1,086,889	$ 940,712	$ 884,333
Operating income	2,883	19,720	137,954	85,843	9,647	18,878	109,230	72,564	246,400	210,319	185,383
Net income (loss)	$ (4,180)	$ 6,890	$ 76,110	$ 45,286	$ 1,654	$ 5,373	$ 58,259	$ 36,812	$ 124,106	$ 102,098	$ 87,898