Consolidated Balance Sheets (Parenthetical) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	50,000,000	50,000,000
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	64,700,976	63,340,929
Common stock, shares outstanding	64,700,976	63,340,929
General Partners' Capital Account, Units Issued	662,373	662,373
General partners' capital, units outstanding	662,373	662,373
Series A cumulative redeemable perpetual preferred units [Member]
Limited partners' capital, common units issued	7,400,000	7,400,000
Limited partners' capital, common units outstanding	7,400,000	7,400,000
Series B cumulative redeemable perpetual preferred units [Member]
Limited partners' capital, common units issued	6,650,000	6,650,000
Limited partners' capital, common units outstanding	6,650,000	6,650,000
Limited partners' common units [Member]
Limited partners' capital, common units issued	64,038,603	62,678,556
Limited partners' capital, common units outstanding	64,038,603	62,678,556
Series A cumulative redeemable perpetual preferred stock [Member]
Preferred stock, shares issued	7,400,000	7,400,000
Preferred stock, shares outstanding	7,400,000	7,400,000
Series B cumulative redeemable perpetual preferred stock [Member]
Preferred stock, shares issued	6,650,000	6,650,000
Preferred stock, shares outstanding	6,650,000	6,650,000