Debt - Warrants (Detail) - shares shares in Millions	3 Months Ended	6 Months Ended
	Jul. 31, 2018	Jul. 31, 2018	Mar. 31, 2013
Class of Warrant or Right [Line Items]
Number of warrants that increased computation of net income per share (in shares)	2	1
Convertible Debt | 0.25% Convertible Senior Notes
Class of Warrant or Right [Line Items]
Contractual interest rate	0.25%	0.25%	0.25%
Shares issues to settle (in shares)	6