Senior Notes (Details) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010
Summary of Fixed Rate Senior Notes
Senior Notes		$ 1,658,600,000	$ 1,468,400,000
Remaining unamortized debt discount		5,400,000	4,400,000
Senior notes		2,139,718,000	2,043,582,000
Debt Instrument, Stated Percentage		5.90%	5.90%
Debt Instrument, Stated Percentage Rate, Minimum		4.75%
Debt Instrument, Stated Percentage Rate, Maximum		9.625%
Debt Instrument Effective Interest Rate Minimum		5.00%
Debt Instrument Effective Interest Rate Maximum		9.90%
Senior Notes maturity date	Apr. 01, 2018
Maturity Date Range, Start		10/1/2012
Maturity Date Range, End		9/1/2020
Senior Convertible Notes Due 2011 [Member]
Summary of Fixed Rate Senior Notes
Senior Convertible Notes			87,500,000
Senior Convertible Notes Due 2012 [Member]
Summary of Fixed Rate Senior Notes
Senior Convertible Notes		178,900,000	194,100,000
Debt Instrument, Stated Percentage		3.00%
Debt Instrument Effective Interest Rate		5.20%
Senior Notes maturity date		Mar. 01, 2012
Senior Convertible Notes Due 2040 [Member]
Summary of Fixed Rate Senior Notes
Senior Convertible Notes		$ 307,600,000	$ 298,000,000
Debt Instrument, Stated Percentage		1.75%
Debt Instrument Effective Interest Rate		5.30%
Senior Notes maturity date		Nov. 01, 2040

Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

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Date when the debt instrument is scheduled to be fully repaid, which may be presented in a variety of ways (year, month and year, day, month and year, quarter, etc.).

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