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Employee Stock Purchase Plan (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Share-based compensation arrangement by share-based payment award, number of shares authorized 39,800,000.0    
Stock-based compensation expense $ 53,154 $ 33,347 $ 33,951
Employee Stock | ANSYS 1996 Employee Stock Purchase Plan [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Share-based compensation arrangement by share-based payment award, number of shares authorized 1,800,000    
Share-based compensation arrangement by share-based payment award, expiration period 6 months    
Share-based compensation arrangement by share-based payment award, maximum employee subscription rate 10.00%    
Maximum number of shares per employee 3,840    
Maximum employee subscription rate, value $ 25    
Share-based compensation arrangement by share-based payment award, shares issued in period 1,500,000    
Stock-based compensation expense $ 1,200 $ 1,200 $ 1,000
Maximum [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Share-based compensation arrangement by share-based payment award, expiration period 10 years    
Maximum [Member] | Employee Stock | ANSYS 1996 Employee Stock Purchase Plan [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Eligibility, ownership percentage of voting power of all classes of stock of the company 5.00%    
Purchase price of common stock, percent 90.00%