Net Income Per Share - Computation of Basic and Diluted Net Income Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011
Disclosure Computation Of Basic And Diluted Net Loss Per Share [Abstract]
Net income attributable to Mentor Graphics shareholders	$ 30,641	$ 24,071	$ 76,990	$ 26,052
Weighted average common shares used to calculate basic net income per share (in shares)	111,575	109,501	110,454	110,423
Employee stock options, restricted stock units and employee stock purchase plan (in shares)	3,146	2,062	3,130	2,758
Weighted average common and potential common shares used to calculate diluted net income per share (in shares)	114,721	111,563	113,584	113,181
Basic net income per share attributable to Mentor Graphics shareholders (in dollars per share)	$ 0.27	$ 0.22	$ 0.70	$ 0.24
Diluted net income per share attributable to Mentor graphics shareholders (in dollars per share)	$ 0.27	$ 0.22	$ 0.68	$ 0.23