Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contracts with Customers
Revenue	$ 344,065	$ 346,419	$ 341,599
Receivables, net	61,760	68,865
Contract Assets - Current	455	656
Contract Assets - Noncurrent	3,777	3,825
Contract Assets - Total	4,232	4,481
Contract Liabilities - Current	5,329	5,513
Contract Liabilities - Noncurrent	6,339	9,844
Contract Liabilities - Total	11,668	15,357
Commodity Swap And Customer Contract [Member]
Revenue from Contracts with Customers
Total gain (loss)	(6,600)	10,300	0
Volume -Related
Revenue from Contracts with Customers
Revenue	273,535	286,684	264,880
Contract Liabilities - Current	3,500	3,500
Volume -Related \| Commodity Swap And Customer Contract [Member]
Revenue from Contracts with Customers
Total gain (loss)	(6,600)	10,300
Station construction sales
Revenue from Contracts with Customers
Revenue	23,120	25,501	51,854
Contract Liabilities - Current	1,800	2,000
AFTC
Revenue from Contracts with Customers
Revenue	47,123	26,729
Compressor sales
Revenue from Contracts with Customers
Revenue			23,527
Other
Revenue from Contracts with Customers
Revenue	$ 287	$ 7,505	$ 1,338

Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

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