Revenues are primarily derived from contracts with customers with terms typically ranging from one to eight years in length and are measured by the amount of consideration we expect to receive in exchange for transferring our products. Revenues are recognized as each performance obligation within the contract is satisfied; this occurs with the transfer of control of our product in accordance with delivery methods as defined in the underlying contract. Performance obligations do not extend beyond one year. Transfer of control to customers generally occurs when products leave our facilities or at other predetermined control transfer point. We account for shipping and handling activities that occur after control of the related good transfers as a cost of fulfillment instead of a separate performance obligation.

We disaggregate revenues by major source consistent with our segment reporting. See Note 18 for further disaggregation of revenue.

Accounts receivable as presented in the consolidated balance sheets are related to our contracts and are recorded when the right to consideration becomes likely at the amount management expects to collect. Accounts receivable do not bear interest if paid when contractually due, and payments are generally due within thirty to forty-five days of invoicing. We typically do not record contract assets, as the transfer of control of our products results in an unconditional right to receive consideration.

We enter into certain supply agreements with customers that include provisions requiring payment at the inception of the supply agreement. Deferred revenue is recorded when payment is received in advance of the performance obligation. Changes in deferred revenue were as follows:

	Three Months Ended March 31,
	2020
	(in thousands)
Beginning balance	$	13,194
Deferral of revenue		21,000
Recognition of unearned revenue		(9,495	)
Ending balance	$	24,699

At March 31, 2020 and December 31, 2019, respectively, deferred revenue balances of $19.6 million and $7.8 million were recorded as current liabilities. At March 31, 2020 and December 31, 2019, respectively, deferred revenue balances of $5.1 million and $5.4 million were recorded in other non-current liabilities.

At March 31, 2020 and December 31, 2019, respectively, we did not have any customers whose accounts receivable balance exceeded 10% of total accounts receivable.

In the three months ended March 31, 2020 we did not have any customers that exceeded 10% of revenues. In the three months ended March 31, 2019, one customer in our Energy segment exceeded 10% of revenues and accounted for 11% of revenues in such period.