Fair Value Measurements (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)	$ 63,497,000		$ 61,073,000
Derivative instruments (Liabilities)	(33,732,000)		(23,792,000)
Total	29,765,000		37,281,000
Fair value amount of transfers in or out of Levels 1 or 2	0	0
Fair values of Convertible Senior Notes	73,100,000
Fair values of Senior Notes	628,000,000
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)
Derivative instruments (Liabilities)
Total
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)	63,497,000		61,073,000
Derivative instruments (Liabilities)	(33,732,000)		(23,792,000)
Total	29,765,000		37,281,000
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)
Derivative instruments (Liabilities)
Total