Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Restricted cash	$ 85,002		$ 85,002		$ 0
Derivative instruments	203,160		203,160		160,452
Derivative assets	240,856		240,856		208,325
Derivative liabilities	(37,696)		(37,696)		(47,873)
Total derivative instruments	203,160		203,160		160,452
Balance at beginning of period	85,580	48,394	97,677	24,720
Total gains or losses (realized/unrealized): Included in earnings	11,682	17,743	27,592	33,787
Total gains or losses (realized/unrealized): Included in other comprehensive income (loss)	3,996	3,151	(11,310)	22,390
Purchases, issuances, and settlements:
Purchases	0	0	0	0
Issuances	0	0	0	0
Settlements	(11,863)	(15,722)	(24,706)	(27,331)
Transfers into/out of Level 3	0	0	142	0
Balance at end of period	89,395	53,566	89,395	53,566
Change in unrealized gains (losses) included in earnings relating to derivatives still held as of June 30	(181)	2,021	2,886	6,456
Carrying Amount [Member]
Cash and cash equivalents	13,279		13,279		16,055
Restricted cash	85,002		85,002		0
Unsecured revolving credit facility	544,200		544,200		421,200
Senior notes	672,400		672,400		673,000
Derivative instruments	203,160		203,160		160,452
Total derivative instruments	203,160		203,160		160,452
Fair Value [Member]
Cash and cash equivalents	13,279		13,279		16,055
Restricted cash	85,002		85,002		0
Unsecured revolving credit facility	544,200		544,200		421,200
Senior notes	770,435		770,435		761,372
Derivative instruments	203,160		203,160		160,452
Total derivative instruments	203,160		203,160		160,452
Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Derivative instruments	0		0		0
Derivative assets	0		0		0
Derivative liabilities	0		0		0
Total derivative instruments	0		0		0
Significant Observable Inputs, Level 2 [Member]
Derivative instruments	113,765		113,765		62,775
Derivative assets	143,325		143,325		86,883
Derivative liabilities	(29,560)		(29,560)		(24,108)
Total derivative instruments	113,765		113,765		62,775
Significant Unobservable Inputs, Level 3 [Member]
Derivative instruments	89,395		89,395		97,677
Derivative assets	97,531		97,531		121,442
Derivative liabilities	(8,136)		(8,136)		(23,765)
Total derivative instruments	89,395		89,395		97,677
7.35% Senior Notes due 2017 [Member]
Debt instrument, interest rate, stated percentage	7.35%		7.35%
7.5% Senior Notes due 2018 [Member]
Debt instrument, interest rate, stated percentage	7.50%		7.50%
Debt instrument maturity			2018
Yield, Publicly-traded debt market	4.80%		4.80%		5.20%
7.125% Senior Notes due 2017 [Member]
Debt instrument, interest rate, stated percentage	7.125%		7.125%
7.15% Senior Notes due 2018 [Member]
Debt instrument, interest rate, stated percentage	7.15%		7.15%
Parent [Member]
Restricted cash	$ 85,002		$ 85,002

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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Issues that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

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The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

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