Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Assets:
Commodity derivatives	$ 6,589
Total	6,589
Liabilities:
Commodity derivatives	15,093
Interest-rate swap derivatives	80,452
Total	95,545
Level 1 [Member]
Assets:
Commodity derivatives	0
Total	0
Liabilities:
Commodity derivatives	0
Interest-rate swap derivatives	0
Total	0
Level 2 [Member]
Assets:
Commodity derivatives	6,589
Total	6,589
Liabilities:
Commodity derivatives	15,093
Interest-rate swap derivatives	80,452
Total	95,545
Level 3 [Member]
Assets:
Commodity derivatives	0
Total	0
Liabilities:
Commodity derivatives	0
Interest-rate swap derivatives	0
Total	$ 0