Net Income (Loss) Per Share - Computation of Basic and Diluted Net Loss Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Apr. 30, 2012	Apr. 30, 2011
Disclosure Computation Of Basic And Diluted Net Loss Per Share [Abstract]
Net income (loss) attributable to Mentor Graphics shareholders	$ 28,182	$ (2,353)
Weighted average common shares used to calculate basic net income (loss) per share (in shares)	109,907	111,769
Employee stock options, restricted stock units and employee stock purchase plan (in shares)	3,336	0
Weighted average common and potential common shares used to calculate diluted net income (loss) per share (in shares)	113,243	111,769
Basic net income (loss) per share attributable to Mentor Graphics shareholders (in dollars per share)	$ 0.26	$ (0.02)
Diluted net income (loss) per share attributable to Mentor graphics shareholders (in dollars per share)	$ 0.25	$ (0.02)