Debt - Schedule of Convertible Senior Notes (Detail) - USD ($) $ in Thousands	Jul. 31, 2015	Jan. 31, 2015	Mar. 31, 2013
Convertible Senior Notes
Debt Instrument [Line Items]
Net carrying amount	$ 1,082,799	$ 1,070,692
Convertible Debt | 0.25% Convertible Senior Notes due April 1, 2018
Debt Instrument [Line Items]
Principal	1,150,000	1,150,000
Less: debt discount, net	$ (67,201)	$ (79,308)
Interest percentage of convertible senior notes	0.25%		0.25%
Effective interest rates of Senior Notes	2.53%