Fair Value Measurements (Details) (USD $)	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)	$ 72,683,000		$ 61,073,000
Derivative instruments (Liabilities)	(14,088,000)		(23,792,000)
Total	58,595,000		37,281,000
Fair value amount of transfers in or out of Levels 1 or 2	0	0
Fair values of Convertible Senior Notes	73,300,000		73,000,000
Fair values of Senior Notes	627,000,000		606,000,000
Level 1 [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]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)	72,683,000		61,073,000
Derivative instruments (Liabilities)	(14,088,000)		(23,792,000)
Total	58,595,000		37,281,000
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments (Assets)	0		0
Derivative instruments (Liabilities)	0		0
Total	$ 0		$ 0