Subsequent Events (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Subsequent Events [Line Items]
Proceeds from the issuance of preferred units	$ 290,971,000	$ 238,842,000	$ 0
Stock Redeemed Or Called During Period Value	72,248,000		35,600,000
Available-for-sale Securities, Gross Realized Gain (Loss)	3,582,000	5,020,000	22,604,000
Repurchase Of Equity [Member] | Series F Preferred Stock [Member]
Subsequent Events [Line Items]
Preferred Stock Dividend Rate Percentage	6.75%
Subsequent Event [Member] | JC Penney [Member]
Subsequent Events [Line Items]
Number of shares sold, shares	10,000,000
Investment Owned Balance Shares	18,584,010
Sales price, per share	$ 16.03
Proceeds from Sale of Available-for-sale Securities, Equity	160,300,000
Available-for-sale Securities, Gross Realized Gain (Loss)	(36,800,000)
Subsequent Event [Member] | Series H Preferred Stock [Member]
Subsequent Events [Line Items]
Preferred Stock Dividend Rate Percentage	6.75%
Subsequent Event [Member] | Series F Preferred Shares And Series H Preferred Shares [Member]
Subsequent Events [Line Items]
Stock Redeemed Or Called During Period Value	262,500,000
Subsequent Event [Member] | Issuance Of Equity [Member] | Series L Preferred Stock [Member]
Subsequent Events [Line Items]
Preferred Stock, Par or Stated Value Per Share	$ 25.00
Sale of Stock, Number of Shares Issued in Transaction	12,000,000
Proceeds from the issuance of preferred units	$ 290,853,000
Preferred Stock Dividend Rate Percentage	5.40%
Preferred Stock Redemption Price Per Share	$ 25.00