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		6 Months Ended
	Jul. 31, 2012	Jul. 31, 2011	Jul. 31, 2012	Jul. 31, 2011
Disclosure Computation Of Basic And Diluted Net Loss Per Share [Abstract]
Net income (loss) attributable to Mentor Graphics shareholders	$ 18,167	$ 4,334	$ 46,349	$ 1,981
Weighted average common shares used to calculate basic net income (loss) per share (in shares)	109,875	110,027	109,891	110,888
Employee stock options, restricted stock units and employee stock purchase plan (in shares)	3,171	2,817	3,187	3,004
Weighted average common and potential common shares used to calculate diluted net income (loss) per share (in shares)	113,046	112,844	113,078	113,892
Basic net income (loss) per share attributable to Mentor Graphics shareholders (in dollars per share)	$ 0.17	$ 0.04	$ 0.42	$ 0.02
Diluted net income (loss) per share attributable to Mentor graphics shareholders (in dollars per share)	$ 0.16	$ 0.04	$ 0.41	$ 0.02