non-renewal of contracts, our business, financial condition, cash flows and results of operations could be materially adversely affected.
As the COVID-19 pandemic and government responses are rapidly evolving, the extent of the impact on domestic coal companies remains unknown. There is considerable uncertainty regarding the extent and duration of governmental and other measures implemented to try to slow the spread of the virus, such as large-scale travel bans and restrictions, border closures, quarantines, stay-at-home orders and business and government shutdowns. Restrictions of this nature have caused, and likely will continue to cause, us, our suppliers and other business counterparties to experience operational delays and delays in the delivery of materials and supplies that are sourced from around the globe, and have caused, and likely will continue to cause, milestones or deadlines relating to various projects to be missed. We have also modified certain business and workforce practices (including those related to employee travel, employee work locations, and cancellation of physical participation in meetings, events and conferences) to conform to government restrictions and best practices encouraged by governmental and regulatory authorities. However, the quarantine of personnel or the inability to access our facilities could adversely affect our operations.
We cannot predict the full impact that COVID-19 will have on our business, cash flows, liquidity, financial condition and results of operations at this time, due to numerous uncertainties. The ultimate impacts will depend on future developments, including, among others, the consequences of governmental and other measures designed to slow the spread of the virus, the development of effective treatments, the duration of the outbreak, actions taken by governmental authorities, customers, suppliers and other third parties, workforce availability, and the timing and extent to which normal economic and operating conditions resume.
Our properties have not yet been fully developed into producing coal mines and, if we experience any development delays or cost increases or are unable to complete the construction of our facilities, our business, financial condition and results of operations could be adversely affected.
We have not completed development plans for all of our coal properties, and do not expect to have full annual production from all of our properties until market conditions permit us to resume and complete these development plans. We expect to incur significant capital expenditures until we have completed the development of our properties. In addition, the development of our properties involves numerous regulatory, environmental, political and legal uncertainties that are beyond our control and that may cause delays in, or increase the costs associated with, their completion. Accordingly, we may not be able to complete the development of the properties on schedule, at the budgeted cost or at all, and any delays beyond the expected development periods or increased costs above those expected to be incurred could have a material adverse effect on our business, financial condition, results of operations, cash flows and ability to pay dividends to our stockholders.
If we are unable to complete or are substantially delayed in completing the development of any of our properties, our business, financial condition, results of operations cash flows and ability to pay dividends to our stockholders could be adversely affected.
We have customer concentration, so the loss of, or significant reduction in, purchases by our largest coal customers could adversely affect our business, financial condition, results of operations and cash flows.
We are exposed to risks associated with an increasingly concentrated customer base both domestically and globally. We derive a significant portion of our revenues from three customers, which accounted for approximately 33%, 24% and 13% of our total revenue, respectively, aggregating approximately 70% of our total revenue for the 12 months ended December 31, 2020. The balance due from these customers at December 31, 2020 was approximately 46% of total accounts receivable.
In 2020, two of these customers declared force majeure with respect to or curtailed their contractual obligations to purchase metallurgical coal from us due to the economic impacts of COVID-19. These reductions and delays lowered our total contracted sales volumes for 2020 by approximately 10% or almost 200,000 tons. Based on the current economic environment, it is possible that a portion of our contractual commitments for 2021 could be similarly delayed or curtailed, and we are not currently able to anticipate the impact such curtailments or delays could have on our business, financial condition, results of operations or cash flows.