Selected Quarterly Financial Data (Unaudited) (Details) (USD $)	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013 Plant	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Data (Unaudited)
Revenue	$ 161,133,000us-gaap_SalesRevenueNet	$ 153,163,000us-gaap_SalesRevenueNet	$ 145,694,000us-gaap_SalesRevenueNet	$ 121,046,000us-gaap_SalesRevenueNet	$ 118,870,000us-gaap_SalesRevenueNet	$ 115,808,000us-gaap_SalesRevenueNet	$ 125,453,000us-gaap_SalesRevenueNet	$ 106,062,000us-gaap_SalesRevenueNet	$ 581,036,000us-gaap_SalesRevenueNet	$ 466,193,000us-gaap_SalesRevenueNet	$ 387,493,000us-gaap_SalesRevenueNet
Gross profit	62,257,000us-gaap_GrossProfit	56,711,000us-gaap_GrossProfit	54,119,000us-gaap_GrossProfit	38,131,000us-gaap_GrossProfit	38,015,000us-gaap_GrossProfit	37,157,000us-gaap_GrossProfit	47,458,000us-gaap_GrossProfit	35,422,000us-gaap_GrossProfit
Net income	18,925,000us-gaap_IncomeLossAttributableToParent	18,103,000us-gaap_IncomeLossAttributableToParent	17,752,000us-gaap_IncomeLossAttributableToParent	6,939,000us-gaap_IncomeLossAttributableToParent	11,359,000us-gaap_IncomeLossAttributableToParent	10,035,000us-gaap_IncomeLossAttributableToParent	27,896,000us-gaap_IncomeLossAttributableToParent	14,733,000us-gaap_IncomeLossAttributableToParent	61,719,000us-gaap_IncomeLossAttributableToParent	64,023,000us-gaap_IncomeLossAttributableToParent	10,509,000us-gaap_IncomeLossAttributableToParent
Income per common unit - basic (in dollars per unit)	$ 0.27	$ 0.26	$ 0.26	$ 0.09	$ 0.19	$ 0.16	$ 0.52	$ 0.31	$ 0.89	$ 1.18	$ 0.14
Income per common unit - diluted (in dollars per unit)	$ 0.27	$ 0.26	$ 0.26	$ 0.09	$ 0.19	$ 0.16	$ 0.52	$ 0.31	$ 0.89	$ 1.18	$ 0.14
Revenue with no incremental costs due to the termination of contracts							6,500,000
Gain on sale of property, plant and equipment due to the termination of contracts							6,800,000us-gaap_GainLossOnDispositionOfAssets		2,466,000us-gaap_GainLossOnDispositionOfAssets	10,140,000us-gaap_GainLossOnDispositionOfAssets	689,000us-gaap_GainLossOnDispositionOfAssets
Number of natural gas processing plants for which purchase options exercised by customer							2
Long-lived asset impairment charge									$ 12,810,000	$ 5,350,000	$ 29,560,000

Number of natural gas processing plants for which purchase options were exercised by customer.

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Amount of revenue recognized with no incremental costs due to termination of contracts resulting from exercise of purchase options.

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Amount of gain (loss) on sale or disposal of property, plant and equipment assets, excluding oil and gas property and timber property.

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The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

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Per unit of ownership amount after tax of income (loss) available to limited partnership (LP) unit-holder and units that would have been outstanding assuming the issuance of limited partner units for dilutive potential units outstanding.

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Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

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