Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012
Assets:
Commodity derivatives	$ 14,890
Total	14,890
Liabilities:
Commodity derivatives	22,564
Interest-rate swap derivatives	84,746
Total	107,310
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	14,890
Total	14,890
Liabilities:
Commodity derivatives	22,564
Interest-rate swap derivatives	84,746
Total	107,310
Level 3 [Member]
Assets:
Commodity derivatives	0
Total	0
Liabilities:
Commodity derivatives	0
Interest-rate swap derivatives	0
Total	$ 0