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Employee Benefit Plans - Defined Contribution Plan (Details) - USD ($)
$ in Thousands
12 Months Ended
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%