Computation of Basic and Diluted Net Loss Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Jan. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Jan. 31, 2011	Oct. 31, 2010	Jul. 31, 2010	Apr. 30, 2010	Jan. 31, 2012	Jan. 31, 2011	Jan. 31, 2010
Earnings Per Share Disclosure [Line Items]
Net income (loss) attributable to Mentor Graphics shareholders	$ 57,820	$ 24,071	$ 4,334	$ (2,353)	$ 50,599	$ 15,257	$ (14,247)	$ (23,025)	$ 83,872	$ 28,584	$ (21,889)
Weighted average common shares used to calculate basic net income (loss) per share									110,138	107,743	96,474
Employee stock options, restricted stock units, and employee stock purchase plans									2,777	2,118
Weighted average common and potential common shares used to calculate diluted net income (loss) per share									112,915	109,861	96,474
Net income (loss) per share attributable to Mentor Graphics shareholders:
Basic	$ 0.53	$ 0.22	$ 0.04	$ (0.02)	$ 0.46	$ 0.14	$ (0.13)	$ (0.22)	$ 0.76	$ 0.27	$ (0.23)
Diluted	$ 0.52	$ 0.22	$ 0.04	$ (0.02)	$ 0.45	$ 0.14	$ (0.13)	$ (0.22)	$ 0.74	$ 0.26	$ (0.23)