Equity (Details) (USD $)	3 Months Ended	12 Months Ended			110 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011
Treasury Stock, Shares, Acquired	10,183				33,123,122
Treasury Stock, Value, Acquired, Cost Method	$ 1,100,000				$ 788,000,000
Treasury Stock Acquired, Average Cost Per Share					$ 23.79
Treasury Stock, Shares, Retired		702,106			27,537,600
Treasury Stock, Retired, Cost Method, Amount		0	0		466,200,000
Treasury Stock, Shares		1,408,334	4,218,726		1,408,334
Treasury shares, Remaining shares authorized to be purchased		11,145,344			11,145,344
Common Stock, Dividends, Per Share, Cash Paid		$ 0.25
Dividends		(46,027,000)	(39,791,000)	(26,416,000)
Common Stock, Dividends, Per Share, Declared		$ 0.28
Warrants Settled		6,600,000
Class of Warrant or Right Average Excercise Price		$ 59.84
Warrants Settled Cash		$ 219,500,000
Shares Issued to Settle Warrants		706,395