Partners' Equity, Allocations and Cash Distributions (Details 2) (USD $) In Thousands, except Per Share data, unless otherwise specified	0 Months Ended												12 Months Ended			9 Months Ended	15 Months Ended	9 Months Ended	15 Months Ended			9 Months Ended	15 Months Ended	9 Months Ended		15 Months Ended	9 Months Ended	15 Months Ended	9 Months Ended	15 Months Ended	9 Months Ended	15 Months Ended	9 Months Ended			15 Months Ended	9 Months Ended
	Feb. 14, 2013	Nov. 14, 2012	Aug. 14, 2012	May 15, 2012	Feb. 14, 2012	Nov. 14, 2011	Aug. 12, 2011	May 13, 2011	Feb. 14, 2011	Nov. 12, 2010	Aug. 13, 2010	May 14, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2011 Common unitholders First order of cash distribution	Dec. 31, 2012 Common unitholders First order of cash distribution	Sep. 30, 2011 Common unitholders Second order of cash distribution	Dec. 31, 2012 Common unitholders Second order of cash distribution	Dec. 31, 2012 Common unitholders Third order of cash distribution	Dec. 31, 2012 Common unitholders Fourth order of cash distribution	Sep. 30, 2011 Common unitholders Thereafter order of cash distribution	Dec. 31, 2012 Common unitholders Thereafter order of cash distribution	Sep. 30, 2011 Subordinated unitholders Third order of cash distribution	Sep. 30, 2011 General partner First order of cash distribution	Dec. 31, 2012 General partner First order of cash distribution	Sep. 30, 2011 General partner Second order of cash distribution	Dec. 31, 2012 General partner Second order of cash distribution	Sep. 30, 2011 General partner Third order of cash distribution	Dec. 31, 2012 General partner Third order of cash distribution	Sep. 30, 2011 General partner Fourth order of cash distribution	Dec. 31, 2012 General partner Fourth order of cash distribution	Sep. 30, 2011 General partner Fifth order of cash distribution	Sep. 30, 2011 General partner Sixth order of cash distribution	Sep. 30, 2011 General partner Thereafter order of cash distribution	Dec. 31, 2012 General partner Thereafter order of cash distribution	Sep. 30, 2011 Common and subordinated unitholders Fourth order of cash distribution	Sep. 30, 2011 Common and subordinated unitholders Fifth order of cash distribution	Sep. 30, 2011 Common and subordinated unitholders Sixth order of cash distribution	Sep. 30, 2011 Common and subordinated unitholders Thereafter order of cash distribution
Partners' Equity, Allocations and Cash Distributions
Percentage of operating surplus distributed to unitholders																98.00%	98.00%	98.00%	98.00%	85.00%	75.00%	50.00%	50.00%	98.00%	2.00%	2.00%	2.00%	2.00%	2.00%	15.00%	2.00%	25.00%	15.00%	25.00%	50.00%	50.00%	98.00%	85.00%	75.00%	50.00%
Summary of distributions per unit
Distribution per Limited Partner Unit (in dollars per unit)	$ 0.5125	$ 0.5075	$ 0.5025	$ 0.4975	$ 0.4925	$ 0.4875	$ 0.4825	$ 0.4775	$ 0.4725	$ 0.4675	$ 0.4625	$ 0.4625	$ 2.0000	$ 1.9200	$ 1.8550				$ 0.4025	$ 0.4375	$ 0.525							$ 0.4025		$ 0.4375	$ 0.4025	$ 0.525	$ 0.4375	$ 0.525			$ 0.4025	$ 0.4375	$ 0.525
Total Distribution	$ 23,300	$ 23,000	$ 22,800	$ 22,500	$ 19,600	$ 19,300	$ 19,100	$ 16,200	$ 16,000	$ 15,700	$ 11,600	$ 11,600	$ 87,866	$ 70,636	$ 50,490