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		9 Months Ended
	Oct. 31, 2015	Oct. 31, 2014	Oct. 31, 2015	Oct. 31, 2014
Disclosure Computation Of Basic And Diluted Net Income Per Share [Abstract]
Net income attributable to Mentor Graphics shareholders	$ 14,679	$ 21,030	$ 36,006	$ 32,651
Adjustment to redemption value of noncontrolling interest with redemption feature	133	(267)	258	1,295
Adjusted net income attributable to Mentor Graphics shareholders, basic	14,812	20,763	36,264	33,946
Adjustment for convertible debt interest, net of tax to be forfeited upon conversion of 4.00% Debentures	0	0	519	0
Adjusted net income attributable to Mentor Graphics shareholders, diluted	$ 14,812	$ 20,763	$ 36,783	$ 33,946
Weighted average common shares used to calculate basic net income per share	117,759	114,405	116,787	114,399
Employee stock options, restricted stock units, employee stock purchase plan and convertible debt	2,382	2,310	5,176	2,529
Weighted average common and potential common shares used to calculate diluted net income per share	120,141	116,715	121,963	116,928
Basic net income per share	$ 0.13	$ 0.18	$ 0.31	$ 0.30
Diluted net income per share	$ 0.12	$ 0.18	$ 0.30	$ 0.29
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest, Net Of Tax, To Be Forfeited Upon Conversion	$ 519	$ 519	$ 0	$ 519
Antidilutive securities excluded from computation of earnings per share, amount	2,695	2,031	0	1,665
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	0	1,657	0	1,138
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	0	20	0	51
Convertible Debt [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	2,695	354	0	476