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, 2016	Oct. 31, 2015	Oct. 31, 2016	Oct. 31, 2015
Disclosure Computation Of Basic And Diluted Net Income Per Share [Abstract]
Net income attributable to Mentor Graphics shareholders	$ 41,762	$ 14,679	$ 31,763	$ 36,006
Adjustment to redemption value of noncontrolling interest with redemption feature	0	133	0	258
Adjusted net income attributable to Mentor Graphics shareholders, basic	41,762	14,812	31,763	36,264
Dilutive Securities, Effect on Basic Earnings Per Share, Dilutive Convertible Securities	519	0	0	519
Adjusted net income attributable to Mentor Graphics shareholders	$ 42,281	$ 14,812	$ 31,763	$ 36,783
Weighted average common shares used to calculate basic net income per share	108,887	117,759	108,442	116,787
Potentially dilutive common shares	5,225	2,382	2,489	5,176
Weighted average common and potentially dilutive common shares used to calculate diluted net income per share	114,112	120,141	110,931	121,963
Basic net income per share	$ 0.38	$ 0.13	$ 0.29	$ 0.31
Diluted net income per share	$ 0.37	$ 0.12	$ 0.29	$ 0.30
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest, net of tax, to be forfeited upon conversion	$ 0	$ 519	$ 519	$ 0
Antidilutive securities excluded from computation of earnings per share, amount	0	2,695	2,349	0
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	0	1,278	0
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	2,695	1,071	0