Net Income (Loss) Per Share - Computation of Basic and Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 31, 2016	Jul. 31, 2015	Jul. 31, 2016	Jul. 31, 2015
Disclosure Computation Of Basic And Diluted Net Income Per Share [Abstract]
Net income (loss) attributable to Mentor Graphics shareholders	$ 3,437	$ 31,212	$ (9,999)	$ 21,327
Adjustment to redemption value of noncontrolling interest with redemption feature	0	(144)	0	125
Adjusted net income (loss) attributable to Mentor Graphics shareholders	$ 3,437	$ 31,068	$ (9,999)	$ 21,452
Weighted average common shares used to calculate basic net income (loss) per share	107,374	116,584	108,215	116,296
Potentially dilutive common shares	2,500	2,784	0	2,690
Weighted average common and potentially dilutive common shares used to calculate diluted net income (loss) per share	109,874	119,368	108,215	118,986
Basic net income (loss) per share	$ 0.03	$ 0.27	$ (0.09)	$ 0.18
Diluted net income (loss) per share	$ 0.03	$ 0.26	$ (0.09)	$ 0.18
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest, net of tax, to be forfeited upon conversion	$ 2,074	$ 2,075	$ 2,074	$ 2,075
Antidilutive securities excluded from computation of earnings per share, amount	1,526	2,825	4,495	3,072
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	28	0	2,265	11
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	0	1,249	0
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	742	0	776	572
Convertible Debt [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	756	2,825	205	2,489