Subsequent Events (Details) (USD $)	0 Months Ended	6 Months Ended			0 Months Ended	6 Months Ended	0 Months Ended	6 Months Ended	0 Months Ended	6 Months Ended	0 Months Ended
	Jul. 02, 2012	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jul. 19, 2012 Series D Cumulative Convertible Preferred Units [Member]	Jun. 30, 2012 Series D Cumulative Convertible Preferred Units [Member]	Jul. 19, 2012 Series E Cumulative Convertible Preferred Units [Member]	Jun. 30, 2012 Series E Cumulative Convertible Preferred Units [Member]	Jul. 19, 2012 Series F Cumulative Convertible Preferred Units [Member]	Jun. 30, 2012 Series F Cumulative Convertible Preferred Units [Member]	Jul. 19, 2012 Common Stock [Member]	Jul. 16, 2012 Accordian Feature [Member] Global Revolving Credit Facility [Member]
Subsequent Event [Line Items]
Common stock, shares issued	11,500,000	110,268,388		106,039,279
Proceeds from issuance of common stock	$ 796,800,000	$ 63,346,000	$ 179,583,000
Dividend and distribution amount					$ 0.343750		$ 0.437500		$ 0.4140625		$ 0.730000
Dividend and distribution payable date					Sep. 28, 2012		Sep. 28, 2012		Sep. 28, 2012		Sep. 28, 2012
Dividend payable to shareholders of record on					Sep. 14, 2012		Sep. 14, 2012		Sep. 14, 2012		Sep. 14, 2012
Annual equivalent rate of dividend and distribution					$ 1.375	$ 1.375	$ 1.750	$ 1.750	$ 1.65625	$ 1.656	$ 2.920
Additional borrowings												$ 300,000,000