Subsequent Events (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2012
Series D Cumulative Convertible Preferred Units [Member]
Subsequent Event [Line Items]
Dividend and distribution amount		$ 0.343750
Dividend and distribution payable date		Dec. 31, 2012
Dividend payable to shareholders of record on		Dec. 14, 2012
Annual equivalent rate of dividend and distribution	$ 1.375	$ 1.375
Series E Cumulative Convertible Preferred Units [Member]
Subsequent Event [Line Items]
Dividend and distribution amount		$ 0.437500
Dividend and distribution payable date		Dec. 31, 2012
Dividend payable to shareholders of record on		Dec. 14, 2012
Annual equivalent rate of dividend and distribution	$ 1.750	$ 1.750
Series F Cumulative Convertible Preferred Units [Member]
Subsequent Event [Line Items]
Dividend and distribution amount		$ 0.414063
Dividend and distribution payable date		Dec. 31, 2012
Dividend payable to shareholders of record on		Dec. 14, 2012
Annual equivalent rate of dividend and distribution	$ 1.656	$ 1.65625
Common Stock [Member]
Subsequent Event [Line Items]
Dividend and distribution amount		$ 0.730000
Dividend and distribution payable date		Jan. 15, 2013
Dividend payable to shareholders of record on		Dec. 14, 2012
Annual equivalent rate of dividend and distribution		$ 2.920