Quarterly Data (unaudited) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Quarterly Financial Data [Abstract]
Revenues	$ 122,216	$ 148,798	$ 395,901	$ 273,797	$ 114,246	$ 143,644	$ 345,802	$ 280,641	$ 940,712	$ 884,333	$ 1,137,366
Operating income	9,647	18,878	109,230	72,564	7,031	25,300	88,447	64,605	210,319	185,383	211,421
Net income (loss)	$ 1,654	$ 5,373	$ 58,259	$ 36,812	$ (1,101)	$ 8,460	$ 47,871	$ 32,668	$ 102,098	$ 87,898	$ 105,196