Fair Value of Financial Instruments (Details) (USD $)	Jun. 30, 2012	Sep. 30, 2011
Assets
Short-term investments		$ 25,829,000
Debt instruments
Estimated fair value of long-term debt	12,500,000	17,500,000
Carrying value of long-term debt	11,500,000	15,900,000
Assets and liabilities measured at fair value on a recurring basis | Level 1
Assets
Cash equivalents	173,451,000	266,842,000
Total assets measured at fair value	173,451,000	266,842,000
Assets and liabilities measured at fair value on a recurring basis | Level 2
Assets
Current derivative assets	648,000	7,466,000
Non-current derivative assets	4,471,000
Total assets measured at fair value	5,119,000	33,295,000
Liabilities
Current derivative liabilities	3,071,000	7,522,000
Non-current derivative liabilities	7,010,000	6,164,000
Total liabilities measured at fair value	10,081,000	13,686,000
Assets and liabilities measured at fair value on a recurring basis | Total
Assets
Cash equivalents	173,451,000	266,842,000
Current derivative assets	648,000	7,466,000
Non-current derivative assets	4,471,000
Total assets measured at fair value	178,570,000	300,137,000
Liabilities
Current derivative liabilities	3,071,000	7,522,000
Non-current derivative liabilities	7,010,000	6,164,000
Total liabilities measured at fair value	10,081,000	13,686,000
Short-term investments -U.S. government agency | Assets and liabilities measured at fair value on a recurring basis | Level 2
Assets
Short-term investments		25,829,000
Short-term investments -U.S. government agency | Assets and liabilities measured at fair value on a recurring basis | Total
Assets
Short-term investments		$ 25,829,000