Short-Term Investments - Schedule of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2014	Jan. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	$ 210,136	$ 25,610
Available-for-sale securities, Gross Unrealized Gains	41	16
Available-for-sale securities, Gross Unrealized Losses	(91)	(1)
Available-for-sale securities, Estimated Fair Value	210,086	25,625
Asset backed-securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	3,700
Available-for-sale securities, Gross Unrealized Gains	1
Available-for-sale securities, Gross Unrealized Losses	(6)
Available-for-sale securities, Estimated Fair Value	3,695
Commercial paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	4,999
Available-for-sale securities, Estimated Fair Value	4,999
Corporate notes and bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	33,525	10,499
Available-for-sale securities, Gross Unrealized Gains	12	9
Available-for-sale securities, Gross Unrealized Losses	(21)	(1)
Available-for-sale securities, Estimated Fair Value	33,516	10,507
U.S. agency obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	136,015	15,111
Available-for-sale securities, Gross Unrealized Gains	20	7
Available-for-sale securities, Gross Unrealized Losses	(52)
Available-for-sale securities, Estimated Fair Value	135,983	15,118
U.S. treasury securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, Amortized Cost	31,897
Available-for-sale securities, Gross Unrealized Gains	8
Available-for-sale securities, Gross Unrealized Losses	(12)
Available-for-sale securities, Estimated Fair Value	$ 31,893