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								12 Months Ended
	Jan. 31, 2013	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 2012	Jan. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2011
Disclosure Computation Of Basic And Diluted Net Loss Per Share [Abstract]
Net Income (Loss) Attributable to Parent	$ 61,746	$ 30,641	$ 18,167	$ 28,182	$ 57,820	$ 24,071	$ 4,334	$ (2,353)	$ 138,736	$ 83,872	$ 28,584
Non-controlling interest adjustment to redemption value									(5,272)	0	0
Net Income (Loss) Attributable to Parent, Diluted									$ 133,464	$ 83,872	$ 28,584
Weighted average common shares used to calculate basic net income per share (in shares)									110,998	110,138	107,743
Employee stock options, restricted stock units and employee stock purchase plan (in shares)									3,019	2,777	2,118
Weighted average common and potential common shares used to calculate diluted net income per share (in shares)									114,017	112,915	109,861
Basic net income per share attributable to Mentor Graphics shareholders (in dollars per share)	$ 0.50	$ 0.27	$ 0.17	$ 0.26	$ 0.53	$ 0.22	$ 0.04	$ (0.02)	$ 1.20	$ 0.76	$ 0.27
Diluted net income per share attributable to Mentor graphics shareholders (in dollars per share)	$ 0.49	$ 0.27	$ 0.16	$ 0.25	$ 0.52	$ 0.22	$ 0.04	$ (0.02)	$ 1.17	$ 0.74	$ 0.26