Cash Distributions (Details) (USD $)	0 Months Ended						3 Months Ended		6 Months Ended
	Jul. 30, 2013	May 15, 2013	Feb. 14, 2013	Nov. 14, 2012	Aug. 14, 2012	May 15, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Summary of distributions per unit
Distribution per Limited Partner Unit (in dollars per unit)		$ 0.5175	$ 0.5125	$ 0.5075	$ 0.5025	$ 0.4975	$ 0.5175	$ 0.4975	$ 1.0300	$ 0.9900
Total Distribution		$ 27,600,000	$ 23,300,000	$ 23,000,000	$ 22,800,000	$ 22,500,000			$ 50,929,000	$ 42,060,000
Cash distribution declared (in dollars per unit)	$ 0.5225
Cash distribution amount declared	$ 27,900,000
Common unitholders | First order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									98.00%
Common unitholders | Second order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									98.00%
Summary of distributions per unit
Distribution per Limited Partner Unit (in dollars per unit)									$ 0.4025
Common unitholders | Third order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									85.00%
Summary of distributions per unit
Distribution per Limited Partner Unit (in dollars per unit)									$ 0.4375
Common unitholders | Fourth order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									75.00%
Summary of distributions per unit
Distribution per Limited Partner Unit (in dollars per unit)									$ 0.525
Common unitholders | Thereafter order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									50.00%
General partner | First order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									2.00%
General partner | Second order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									2.00%
Summary of distributions per unit
Distribution per Limited Partner Unit (in dollars per unit)									$ 0.4025
General partner | Third order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									15.00%
Summary of distributions per unit
Distribution per Limited Partner Unit (in dollars per unit)									$ 0.4375
General partner | Fourth order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									25.00%
Summary of distributions per unit
Distribution per Limited Partner Unit (in dollars per unit)									$ 0.525
General partner | Thereafter order of cash distribution
Cash Distributions
Percentage of operating surplus distributed to unitholders									50.00%