Net Income Per Share - Computation of Basic and Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2015	Jan. 31, 2015	Oct. 31, 2014	Jul. 31, 2014	Apr. 30, 2014	Jan. 31, 2016	Jan. 31, 2015	Jan. 31, 2014
Disclosure Computation Of Basic And Diluted Net Income Per Share [Abstract]
Net income attributable to Mentor Graphics shareholders	$ 60,271	$ 14,679	$ 31,212	$ (9,885)	$ 114,488	$ 21,030	$ 14,172	$ (2,551)	$ 96,277	$ 147,139	$ 155,258
Adjustment of noncontrolling interest to redemption value	$ 0	$ 133	$ (144)	$ 269	$ (1,174)	$ (267)	$ 895	$ 667	258	121	(4,486)
Adjusted net income attributable to Mentor Graphics shareholders									$ 96,535	$ 147,260	$ 150,772
Weighted average common shares used to calculate basic net income per share									116,701	114,635	113,671
Employee stock options, restricted stock units and employee stock purchase plan									2,562	2,443	3,031
Weighted average common and potential common shares used to calculate diluted net income per share									119,263	117,078	116,702
Basic net income per share	$ 0.52	$ 0.13	$ 0.27	$ (0.08)	$ 0.98	$ 0.18	$ 0.13	$ (0.02)	$ 0.83	$ 1.28	$ 1.33
Diluted net income per share	$ 0.51	$ 0.12	$ 0.26	$ (0.08)	$ 0.96	$ 0.18	$ 0.13	$ (0.02)	$ 0.81	$ 1.26	$ 1.29
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Adjustment for convertible debt interest, net of tax to be forfeited upon conversion of 4.00% Debentures									$ 2,074	$ 2,075	$ 2,075
Antidilutive securities excluded from computation of earnings per share, amount									2,046	1,531	1,195
Restricted Stock Units (RSUs)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount									0	18	566
Employee Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount									0	14	518
ESPP Purchase Right [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount									0	887	111
Convertible Debt [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount									2,046	612	0