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		6 Months Ended
	Jul. 31, 2015	Jul. 31, 2014	Jul. 31, 2015	Jul. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Adjustment to redemption value of non-controlling interest with redemption feature	$ (144)	$ 895	$ 125	$ 1,562
Antidilutive securities excluded from computation of earnings per share, amount	0	20	583	701
Net income attributable to Mentor Graphics shareholders	$ 31,212	$ 14,172	$ 21,327	$ 11,621
Adjustment to redemption value of non-controlling interest with redemption feature	(144)	895	125	1,562
Adjusted net income attributable to Mentor Graphics shareholders	$ 31,068	$ 15,067	$ 21,452	$ 13,183
Weighted average common shares used to calculate basic net income per share	116,584	113,868	116,296	114,396
Employee stock options, restricted stock units and employee stock purchase plan	2,784	2,683	2,690	2,564
Weighted average common and potential common shares used to calculate diluted net income per share	119,368	116,551	118,986	116,960
Basic net income per share	$ 0.27	$ 0.13	$ 0.18	$ 0.12
Diluted net income per share	$ 0.26	$ 0.13	$ 0.18	$ 0.11
Convertible Debt Securities [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	2,825	521	2,489	538
Interest, net of tax, to be forfeited upon conversion	$ 2,075	$ 2,075	$ 2,075	$ 2,075