Statement of Cash Flows Supplemental Data (Narratives) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Supplemental Cash Flow Information
Noncash or Part Noncash Acquisition, Value of Liabilities Assumed	$ 8,200,000	$ 3,400,000
Change Asset Retirement Obligations	0	1,200,000	6,842,000
Distributions payable	$ 50,207,000		$ 46,412,000