Fair Value Measurements (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value amount of transfers in or out of Levels 1 or 2	$ 0	$ 0
Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)	36,452,000	61,063,000
Derivative instruments (Liabilities)	(7,291,000)	(23,569,000)
Total	29,161,000	37,494,000
Level 1 [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)	0	0
Derivative instruments (Liabilities)	0	0
Total	0	0
Level 2 [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)	36,452,000	61,063,000
Derivative instruments (Liabilities)	(7,291,000)	(23,569,000)
Total	29,161,000	37,494,000
Level 3 [Member] | Portion at Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)	0	0
Derivative instruments (Liabilities)	0	0
Total	$ 0	$ 0