Note 2 —Revenue from Contracts with Customers

Disaggregation of Revenue

The table below presents the Company’s revenue disaggregated by revenue source (in thousands):


	Years Ended December 31,
	2017		2018		2019
Volume-related (1)	$	264,880	$	286,684	$	273,535
Station construction sales		51,854		25,501		23,120
AFTC		—		26,729		47,123
Compressor sales (2)		23,527		—		—
Other		1,338		7,505		287
Total revenue	$	341,599	$	346,419	$	344,065

(1) Includes changes in fair value of derivative instruments related to the Company’s commodity swap and customer fueling contracts. See Note 1 and Note 8 for more information about these derivative instruments. For the years ended December 31, 2018 and 2019, changes in the fair value of commodity swaps and customer fueling contracts amounted to a gain of $10.3 million and a loss of $6.6 million, respectively.

(2) The Company completed the CEC Combination (as defined in Note 4) during the year ended December 31, 2017 and no longer generates revenue from compressor sales.

Remaining Performance Obligations

Remaining performance obligations represent the transaction price of customer orders for which the work has not been performed. As of December 31, 2019, the aggregate amount of the transaction price allocated to remaining performance obligations was $15.8 million, which related to the Company’s station construction sale contracts. The Company expects to recognize revenue on the remaining performance obligations under these contracts over the next 12 to 24 months.

For volume-related revenue, the Company has elected to apply an optional exemption, which waives the requirement to disclose the remaining performance obligation for revenue recognized through the ‘right to invoice’ practical expedient.

Costs to Fulfill a Contract

The Company capitalizes costs incurred to fulfill its contracts that (1) relate directly to the contract, (2) are expected to generate resources that will be used to satisfy the Company’s performance obligations under the contract, and (3) are expected to be recovered through revenue generated under the contract. Contract fulfillment costs are recorded to depreciation expense as the Company satisfies its performance obligations over the term of the contract. These costs primarily relate to set-up and other direct installation costs incurred by NG Advantage, for equipment that must be installed on customers’ land before NG Advantage, LLC (“NG Advantage”) is able to deliver CNG to the customer because the customer does not have direct access to the natural gas pipelines. These costs are classified in “Land, property, and equipment, net” in the accompanying consolidated balance sheets. As of December 31, 2018 and 2019, these capitalized costs incurred to fulfill contracts were $9.1 million and $10.2 million with accumulated depreciation of $4.9 million and $6.3 million, respectively, and related depreciation expense of $2.0 million and $2.2 million for the years ended December 31, 2018 and 2019, respectively.

Contract Balances

The timing of revenue recognition, billings and cash collections results in billed accounts receivable, unbilled receivables (contract assets), and customer advances and deposits (contract liabilities) in the accompanying consolidated balance sheets. Changes in the contract asset and liability balances during the year ended December 31, 2019, were not materially affected by any factors outside the normal course of business.

As of December 31, 2018 and 2019, the Company’s contract balances were as follows (in thousands):


	2018		2019
Accounts receivable, net	$	68,865	$	61,760

Contract assets - current	$	656	$	455
Contract assets - non-current		3,825		3,777
Contract assets - total	$	4,481	$	4,232

Contract liabilities - current	$	5,513	$	5,329
Contract liabilities - non-current		9,844		6,339
Contract liabilities - total	$	15,357	$	11,668

Accounts Receivable, Net

“Accounts receivable, net” in the accompanying consolidated balance sheets include amounts billed and currently due from customers. The amounts due are stated at their net estimated realizable value. The Company maintains an allowance for doubtful accounts to provide for the estimated amount of receivables that will not be collected. The allowance is based upon an assessment of customer creditworthiness, historical payment experience, and the age of outstanding receivables.

Contract Assets

Contract assets include unbilled amounts typically resulting from the Company’s station construction sale contracts, when the cost-to-cost method of revenue recognition is utilized and revenue recognized exceeds the amount billed to the customer, and right to payment is not just subject to the passage of time. Amounts may not exceed their net realizable value. Contract assets are classified as current or noncurrent based on the timing of billings. The current portion is included in “Prepaid expenses and other current assets” and the noncurrent portion is included in “Notes receivable and other long-term assets, net” in the accompanying consolidated balance sheets.

Contract Liabilities

Contract liabilities consist of billings in excess of revenue recognized from the Company’s station construction sale contracts and payments primarily from a customer of NG Advantage in advance of the performance obligations. Billings in excess of revenue recognized of $2.0 million and $1.8 million and advance payments of $3.5 million and $3.5 million are classified as current as of December 31, 2018 and 2019, respectively. Deferred revenue is classified as current or noncurrent based on when the revenue is expected to be recognized. The current portion and noncurrent portion of deferred revenue are included in “Deferred revenue” and “Other long-term liabilities,” respectively, in the accompanying consolidated balance sheets.

The increase in the contract liabilities balance for the year ended December 31, 2019 is primarily driven by billings in excess of revenue recognized, offset by $5.9 million of revenue recognized related to the Company’s contract liability balances as of December 31, 2018.