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Employee Benefit Plans - Defined Contribution Plan (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Contribution Plan |  |  |  |
Employer contributed by eligible participants | 50.00% | 50.00% | 50.00% |
Pre-tax compensation contributed | 6.00% |  |  |
Defined contribution plan expenses | $ 1,600 | $ 1,200 | $ 500 |
Maximum |  |  |  |
Defined Contribution Plan |  |  |  |
Defined Contribution Plan, Employer Discretionary Contribution Amount |  | $ 1,200 |  | $ 1
Regular employees |  |  |  |
Defined Contribution Plan |  |  |  |
Maximum contribution per employee under the Axcelis Long-Term Investment Plan (as a percent) | 35.00% |  |  |
Highly compensated employees |  |  |  |
Defined Contribution Plan |  |  |  |
Maximum contribution per employee under the Axcelis Long-Term Investment Plan (as a percent) | 16.00% |  |  |

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information. + References No definition available. + Details
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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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