Statement of Cash Flows Supplemental Data (Narratives) (Details) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Supplemental Cash Flow Information
Deferred acquisition fees payable	$ 1,400,000	$ 4,400,000
Change Asset Retirement Obligations	0	1,200,000	6,842,000
Distributions payable	50,689,000	38,500,000	46,412,000
Real estate
Schedule of Investments [Line Items]
Construction In Progress Placed Into Service	26,100,000	114,800,000
Operating real estate
Schedule of Investments [Line Items]
Construction In Progress Placed Into Service	$ 12,600,000