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Employee Stock Purchase Plan Employee Stock Purchase Plan (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]        
Share-based compensation arrangement by share-based payment award, number of shares authorized 30,400,000 30,400,000    
Stock-based compensation expense   $ 33,951 $ 36,861 $ 35,298
Employee Stock | ANSYS 1996 Employee Stock Purchase Plan [Member]        
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]        
Share-based compensation arrangement by share-based payment award, number of shares authorized 1,600,000 1,600,000    
Share-based compensation arrangement by share-based payment award, maximum employee subscription rate 10.00% 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 1,400,000      
Stock-based compensation expense   $ 1,036 $ 876 $ 765
Maximum [Member] | Employee Stock | ANSYS 1996 Employee Stock Purchase Plan [Member]        
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]        
Eligibility, ownership percentage of voting power of all classes of stock of the company 5.00% 5.00%    
Purchase price of common stock, percent   90.00%