Quarterly Data (unaudited) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Quarterly Financial Data [Abstract]
Revenues	$ 114,246	$ 143,644	$ 345,802	$ 280,641	$ 129,671	$ 172,714	$ 484,465	$ 350,516	$ 884,333	$ 1,137,366	$ 1,169,539
Operating income	7,031	25,300	88,447	64,605	8,946	19,486	104,109	78,880	185,383	211,421	189,519
Net income (loss)	$ (1,101)	$ 8,460	$ 47,871	$ 32,668	$ (1,519)	$ 5,478	$ 60,163	$ 41,074	$ 87,898	$ 105,196	$ 90,258