Certain provisions of the partnership agreement (Textuals) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Distribution Made to Member or Limited Partner [Line Items]
Percentage Of Cash Avaliable For Distribution	100.00%
Number of days from end of quarter for distribution	45
General Partner [Member]
Distribution Made to Member or Limited Partner [Line Items]
Incentive Distribution, Distribution	$ 1.4	$ 1.0
General Partner [Member] | Thirteen Percent [Member]
Distribution Made to Member or Limited Partner [Line Items]
Incentive Distibution Excess Per Unit Amount	$ 0.25
Incentive Distribution Percentage Levels	13.00%
General Partner [Member] | Twentythree Percent [Member]
Distribution Made to Member or Limited Partner [Line Items]
Incentive Distibution Excess Per Unit Amount	$ 0.3125
Incentive Distribution Percentage Levels	23.00%
General Partner [Member] | Fortyeight Percent [Member]
Distribution Made to Member or Limited Partner [Line Items]
Incentive Distibution Excess Per Unit Amount	$ 0.375
Incentive Distribution Percentage Levels	48.00%