Employee Stock Purchase Plan Employee Stock Purchase Plan (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Number of shares available for offerings	30,400,000
Employee Stock | ANSYS 1996 Employee Stock Purchase Plan [Member]
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Number of shares available for offerings	1,600,000
Maximum number of shares per employee	3,840
Maximum employee subscription rate, value	$ 25
Shares of common stock issued under Purchase Plan	1,233,385	1,184,082
Stock-based compensation expense	$ 706	$ 650	$ 500
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%
Maximum employee subscription rate	10.00%
Purchase price of common stock, percent	90.00%