Certain provisions of the partnership agreement (Distribution to Unit Holders) (Details Textuals) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Distribution Made To Common And Preferred Units Holders [Line Items]
Distribution Made To Member Or Limited Partner Distribution Date	May 15, 2013	Feb. 14, 2013
Common Stock, Dividends, Per Share, Cash Paid	$ 0.33	$ 0.33
Preferred Stock, Dividends, Per Share, Cash Paid	$ 0.33	$ 0.33
Preferred Stock P I K [Member]
Distribution Made To Common And Preferred Units Holders [Line Items]
Preferred Stock Paid In Kind	0.4