Cash Distributions (Details) (USD $)	0 Months Ended					3 Months Ended
	Apr. 29, 2013	Feb. 14, 2013	Nov. 14, 2012	Aug. 14, 2012	May 15, 2012	Mar. 31, 2013	Mar. 31, 2012
Summary of distributions per unit
Distribution per Limited Partner Unit (in dollars per unit)		$ 0.5125	$ 0.5075	$ 0.5025	$ 0.4975	$ 0.5125	$ 0.4925
Total Distribution		$ 23,331,000	$ 23,000,000	$ 22,800,000	$ 22,500,000	$ 23,331,000	$ 19,580,000
Cash distribution declared (in dollars per unit)	$ 0.5175
Cash distribution amount declared	$ 27,600,000
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%