Financial Instruments and Concentrations of Risk (Details Textual) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) employees	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) employees	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Margin deposits	$ 1,900				$ 3,600				$ 1,900	$ 3,600
Derivative Asset, Fair Value, Amount Offset Against Collateral	500				2,300				500	2,300
Revenue, Net	$ 606,806	$ 545,922	$ 473,089	$ 456,100	$ 402,172	$ 327,764	$ 287,377	$ 314,851	2,081,917	1,332,164	$ 1,455,094
SemGas [Member]
Revenue, Net									$ 222,048	$ 208,042	$ 231,569
Number of Employees, Geographic Area [Member] | Geographic Concentration Risk [Member]
Number of employees | employees	1,220								1,220
Non-US [Member] | Number of Employees, Geographic Area [Member] | Geographic Concentration Risk [Member]
Number of employees	550								550
Mexico and Canada [Member] | Workforce Subject to Collective Bargaining Arrangements [Member] | Unionized Employees Concentration Risk [Member]
Number of employees	130								130
Largest Customer [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member]
Revenue, Net									$ 646,100
Largest Customer [Member] | Accounts Receivable [Member] | Customer Concentration Risk [Member]
Concentration of risk, percentage									29.00%
Two Largest Producers [Member] | Processing volumes [Member] | Product Concentration Risk [Member] | SemGas [Member]
Concentration of risk, percentage									88.00%
Two Largest Producers [Member] | Gathering volumes [Member] | Product Concentration Risk [Member] | SemGas [Member]
Concentration of risk, percentage									91.00%
Agreements With Annual Renewals [Member] | Mexico and Canada [Member] | Workforce Subject to Collective Bargaining Arrangements [Member] | Unionized Employees Concentration Risk [Member]
Number of employees	70								70
Expired Agreements Being Renegotiated [Member] | Mexico and Canada [Member] | Workforce Subject to Collective Bargaining Arrangements [Member] | Unionized Employees Concentration Risk [Member]
Number of employees	55								55
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount	$ 491,100								$ 491,100
Derivative Liability	1,200								1,200
Derivative, Gain (Loss) on Derivative, Net									1,100
Not Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member]
Derivative, Notional Amount	197,700								197,700
Derivative Asset	$ 2,600								2,600
Derivative, Gain (Loss) on Derivative, Net									$ 2,800