Fair Value Measurements - Fair Value Hierarchy for Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jan. 31, 2017	Jan. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	$ 301,266	$ 214,024
Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	20,570	5,970
Corporate notes and bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	92,756	38,327
U.S. agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	87,052	124,586
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	20,718	5,454
U.S. treasury securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	80,170	39,687
Fair value, measurements recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	326,890	256,509
Fair value, measurements recurring [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	20,174	28,087
Fair value, measurements recurring [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total	306,716	228,422
Fair value, measurements recurring [Member] | Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	20,174	28,087
Fair value, measurements recurring [Member] | Money market funds [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	20,174	28,087
Fair value, measurements recurring [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	20,570	5,970
Fair value, measurements recurring [Member] | Commercial paper [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	20,570	5,970
Fair value, measurements recurring [Member] | Corporate notes and bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		11,396
Short-term investments	92,756	38,327
Fair value, measurements recurring [Member] | Corporate notes and bonds [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		11,396
Short-term investments	92,756	38,327
Fair value, measurements recurring [Member] | U.S. agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	5,450	3,002
Short-term investments	87,052	124,586
Fair value, measurements recurring [Member] | U.S. agency obligations [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	5,450	3,002
Short-term investments	87,052	124,586
Fair value, measurements recurring [Member] | Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	20,718	5,454
Fair value, measurements recurring [Member] | Asset-backed securities [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	20,718	5,454
Fair value, measurements recurring [Member] | U.S. treasury securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	80,170	39,687
Fair value, measurements recurring [Member] | U.S. treasury securities [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	$ 80,170	$ 39,687