Preferred Stock of the Company (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended			3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2012 Quarters	Mar. 31, 2011	Dec. 31, 2011	Mar. 31, 2012 Series E Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series E Cumulative Redeemable Preferred Stock [Member]	Mar. 31, 2012 Series F Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series F Cumulative Redeemable Preferred Stock [Member]	Mar. 31, 2012 Series G Cumulative Redeemable Preferred Stock [Member] NumberofTradingDays	Dec. 31, 2011 Series G Cumulative Redeemable Preferred Stock [Member]	Mar. 31, 2012 Cumulative Preferred Stock Subject to Mandatory Redemption [Member]
Preferred Stock of the Company [Abstract]
Preferred stock dividend rate percentage				7.80%	7.80%	7.50%	7.50%	6.875%
Preferred Stock, Shares Outstanding				1,610,000	1,610,000	3,450,000	3,450,000	4,000,000	0
Preferred Stock, Redemption Price Per Share				$ 25	$ 25	$ 25	$ 25
7.80% Series E and 7.50% Series F Cumulative Redeemable Preferred stock, called for redemption (Note 7)	$ 126,500		$ 0
Dividends Payable	26,622		22,692							2,900
Original issuance costs of preferred stock called for redemption (Note 7)	4,918	0
Preferred Stock, Shares Issued				1,610,000	1,610,000	3,450,000	3,450,000	4,000,000	0
Preferred Stock, Liquidation Preference Per Share								$ 25
Net proceeds from Issuance of Series G preferred stock (Note 7)	$ 96,155							$ 96,155
Number of quarters	6
Number of days within a change in control in which preferred stock may be redeemed								120
Redeemable Preferred Stock Shares Issued Upon Conversion Maximum								4,390,000