Shareholders Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended						12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Series A Preferred Stock [Member]	Dec. 31, 2011 Series A Preferred Stock [Member] Convertible Preferred Stock [Member]	Dec. 31, 2010 Series A Preferred Stock [Member] Convertible Preferred Stock [Member]	Dec. 31, 2011 Series E Preferred Stock [Member]	Dec. 31, 2011 Series E Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2010 Series E Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series F Preferred Stock [Member]	Dec. 31, 2011 Series F Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2010 Series F Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series G Preferred Stock [Member]	Dec. 31, 2011 Series G Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2010 Series G Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series H Preferred Stock [Member]	Dec. 31, 2011 Series H Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2010 Series H Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series I Preferred Stock [Member]	Dec. 31, 2011 Series I Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2010 Series I Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series J Preferred Stock [Member]	Dec. 31, 2011 Series J Preferred Stock [Member] Redeemable Preferred Stock [Member]	Dec. 31, 2010 Series J Preferred Stock [Member] Redeemable Preferred Stock [Member]	Apr. 20, 2011 Series J Preferred Stock [Member] Public Offering [Member]	Aug. 05, 2011 Series J Preferred Stock [Member] Public Offering [Member] Redeemable Preferred Stock [Member]	May 05, 2011 Series J Preferred Stock [Member] Public Offering [Member] Redeemable Preferred Stock [Member]	Apr. 20, 2011 Series J Preferred Stock [Member] Public Offering [Member] Redeemable Preferred Stock [Member]	Apr. 20, 2011 Series J Preferred Stock [Member] Over Allotment [Member] Redeemable Preferred Stock [Member]
Class Of Stock [Line Items]
Accumulated other comprehensive income	$ 73,729	$ 73,453
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock Dividend Rate Percentage				6.50%			7.00%			6.75%			6.625%			6.75%			6.625%			6.875%
Preferred shares of beneficial interest: authorized shares	110,000,000	110,000,000			5,750,000	5,750,000		3,450,000	3,450,000		6,000,000	6,000,000		9,200,000	9,200,000		4,600,000	4,600,000		12,050,000	12,050,000		9,850,000	0
Preferred Stock, Value	1,021,660	783,088			1,787	2,057		72,248	72,248		144,720	144,720		193,135	193,135		108,549	108,549		262,379	262,379		238,842	0
Preferred shares of beneficial interest: outstanding shares	42,186,709	32,340,009			36,709	40,009		3,000,000	3,000,000		6,000,000	6,000,000		8,000,000	8,000,000		4,500,000	4,500,000		10,800,000	10,800,000		9,850,000	0
Preferred Stock Liquidation Preference					$ 50.00			$ 25.00			$ 25.00			$ 25.00			$ 25.00			$ 25.00			$ 25.00
Preferred Stock, Dividend Rate, Per-Dollar-Amount					$ 3.25			$ 1.75			$ 1.6875			$ 1.656			$ 1.6875			$ 1.656			$ 1.71875
Preferred Stock, Additional Disclosure [Abstract]
Preferred shares of beneficial interest: issued shares	42,186,709	32,340,009																								1,000,000	800,000	7,000,000	1,050,000
Preferred Stock Par Or Stated Value Per Share																									$ 25
Proceeds from the issuance of shares, Value	238,842		710,226
Stock Redeemed Or Called During Period Value		35,600
Preferred Stock Redemption Price Per Share								$ 25.00			$ 25.00			$ 25.00			$ 25.00			$ 25.00			$ 25.00
Preferred Stock Redemption Discount	5,000	4,382	0
Conversion Rate of Preferred Stock to Common Stock					1.4334
Proceeds from the issuance of Series J preferred shares	$ 238,842	$ 0	$ 0