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, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2016	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2015	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2015
Disclosure Computation Of Basic And Diluted Net Income Per Share [Abstract]
Net income attributable to Mentor Graphics shareholders	$ 123,103	$ 41,762	$ 3,437	$ (13,436)	$ 60,271	$ 14,679	$ 31,212	$ (9,885)	$ 154,866	$ 96,277	$ 147,139
Adjustment of noncontrolling interest to redemption value					$ 0	$ 133	$ (144)	$ 269	0	258	121
Adjusted net income attributable to Mentor Graphics shareholders, basic									154,866	96,535	147,260
Adjustment for convertible debt interest, net of tax to be forfeited upon conversion of 4.00% Debentures									2,074	0	0
Adjusted net income attributable to Mentor Graphics shareholders, diluted									$ 156,940	$ 96,535	$ 147,260
Weighted average common shares used to calculate basic net income per share									108,795	116,701	114,635
Potentially dilutive common shares									5,527	2,562	2,443
Weighted average common and potentially dilutive common shares used to calculate diluted net income per share									114,322	119,263	117,078
Basic net income per share	$ 1.12	$ 0.38	$ 0.03	$ (0.12)	$ 0.52	$ 0.13	$ 0.27	$ (0.08)	$ 1.42	$ 0.83	$ 1.28
Diluted net income per share	$ 1.05	$ 0.37	$ 0.03	$ (0.12)	$ 0.51	$ 0.12	$ 0.26	$ (0.08)	$ 1.37	$ 0.81	$ 1.26
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									$ 0	$ 2,074	$ 2,075
Antidilutive securities excluded from computation of earnings per share, amount									12	2,046	1,531
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									12	0	18
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	0	14
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	0	887
Convertible Debt [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount									0	2,046	612