2. REVENUE

The Corporation recognizes revenue when control of a promised good and/or service is transferred to a customer in an amount that reflects the consideration that the Corporation expects to be entitled to in exchange for that good and/or service.

Performance Obligations

The Corporation identifies a performance obligation for each promise in a contract to transfer a distinct good or service to the customer. As part of its assessment, the Corporation considers all goods and/or services promised in the contract, regardless of whether they are explicitly stated or implied by customary business practices. The Corporation’s contracts may contain either a single performance obligation, including the promise to transfer individual goods or services that are not separately distinct within the context of the respective contracts, or multiple performance obligations. For contracts with multiple performance obligations, the Corporation allocates the overall transaction price to each performance obligation using standalone selling prices, where available, or utilizes estimates for each distinct good or service in the contract where standalone prices are not available.

The Corporation’s performance obligations are satisfied either at a point-in-time or on an over-time basis. Revenue recognized on an over-time basis accounted for approximately 48% and 45% of total net sales for the three months ended March 31, 2019 and 2018, respectively. Typically, over-time revenue recognition is based on the utilization of an input measure used to measure progress, such as costs incurred to date relative to total estimated costs. Revenue recognized at a point-in-time accounted for approximately 52% and 55% of total net sales for the three months ended March 31, 2019 and 2018, respectively. Revenue for these types of arrangements is recognized at the point in time in which control is transferred to the customer, typically based upon the terms of delivery.

Contract backlog represents the remaining performance obligations that have not yet been recognized as revenue. Backlog includes deferred revenue and amounts that will be invoiced and recognized as revenue in future periods. Total backlog was approximately $2.3 billion as of March 31, 2019, of which the Corporation expects to recognize approximately 93% as net sales over the next 12 -36 months. The remainder will be recognized thereafter.

Disaggregation of Revenue

The following table presents the Corporation’s total net sales disaggregated by end market and customer type:



	Three Months Ended March 31,
Total Net Sales by End Market and Customer Type (In thousands)	2019		2018
Defense
Aerospace	$	78,787	$	79,153
Ground	20,758		22,519
Naval	131,088		103,489
Total Defense Customers	$	230,633	$	205,161

Commercial
Aerospace	$	103,221	$	99,404
Power Generation	96,480		98,319
General Industrial	147,980		144,638
Total Commercial Customers	$	347,681	$	342,361

Total	$	578,314	$	547,522


Note: Certain amounts in the prior year have been reclassed to conform to the current year presentation.

Contract Balances

Timing of revenue recognition and cash collection may result in billed receivables, unbilled receivables (contract assets), and deferred revenue (contract liabilities) on the Condensed Consolidated Balance Sheet. The Corporation’s contract assets primarily relate to its rights to consideration for work completed but not billed as of the reporting date. Contract assets are transferred to billed receivables when the rights to consideration become unconditional. This is typical in situations where amounts are billed as work progresses in accordance with agreed-upon contractual terms or upon achievement of contractual milestones. The Corporation’s contract liabilities primarily consist of customer advances received prior to revenue being earned. Revenue recognized during the three months ended March 31, 2019 included in the contract liabilities balance at the beginning of the year was approximately $79 million. Changes in contract assets and contract liabilities as of March 31, 2019, were not materially impacted by any other factors. Contract assets and contract liabilities are reported in the "Receivables, net" and "Deferred revenue" lines, respectively, within the Condensed Consolidated Balance Sheet.