Stock-Based Compensation - Assumptions to Estimate Fair Value of Stock Awards (Details) - Restricted Stock Unit Compensation Expense - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Risk-free interest rate	0.70%	2.50%	2.40%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	2 years 9 months 18 days	2 years 9 months 18 days	2 years 9 months 18 days
Correlation factor	0.77	0.71	0.65
Weighted average fair value per share (in dollars per share)	$ 245.08	$ 238.99	$ 191.76
Ansys Stock Price
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected volatility	25.00%	23.00%	21.00%
Nasdaq Composite Index
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected volatility	18.00%	16.00%	15.00%

Share-based compensation arrangement by share-based payment award, fair value assumptions, correlation factor

No definition available.

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-Publisher FASB
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