Summary of Quarterly Financial Information (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
Quarterly Financial Information [Line Items]
Total revenues	$ 320,355	$ 250,508	$ 213,740	$ 230,035	$ 307,305	$ 238,937	$ 187,934	$ 180,577
Gross margin	275,680	210,262	169,848	185,390	262,596	195,947	152,764	147,198	841,180	758,505	668,176
Operating income (loss)	65,770	25,405	4,844	16,173	57,383	20,641	(10,491)	(14,994)	112,192	52,539	(1,167)
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)
Net income (loss) per share, basic	$ 0.53	$ 0.22	$ 0.04	$ (0.02)	$ 0.46	$ 0.14	$ (0.13)	$ (0.22)	$ 0.76	$ 0.27	$ (0.23)
Net income (loss) per share, diluted	$ 0.52	$ 0.22	$ 0.04	$ (0.02)	$ 0.45	$ 0.14	$ (0.13)	$ (0.22)	$ 0.74	$ 0.26	$ (0.23)