FAIR VALUE MEASUREMENTS - Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Jul. 31, 2017	Nov. 01, 2016	Jul. 31, 2016	Jul. 31, 2015	Jul. 31, 2014
Financial Assets:
Short-term investments:	$ 210,694		$ 85,991
Financial Liabilities:
Convertible preferred stock warrant liability	0	$ 30,812	9,679
Warrant
Financial Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	0		9,679	$ 11,683	$ 5,507
Recurring
Financial Assets:
Total measured at fair value	268,519		138,295
Cash	80,534		46,905
Total cash, cash equivalents and short-term investments	349,053		185,200
Recurring | Warrant
Financial Liabilities:
Convertible preferred stock warrant liability	4,295		9,679
Recurring | Corporate bonds
Financial Assets:
Short-term investments:	160,634		64,360
Recurring | Commercial paper
Financial Assets:
Short-term investments:	36,084		21,631
Recurring | US Government Debt Securities
Financial Assets:
Short-term investments:	13,976
Recurring | Money market funds
Financial Assets:
Cash equivalents:	34,784		47,305
Recurring | Commercial paper
Financial Assets:
Cash equivalents:	23,041		4,999
Recurring | Level I
Financial Assets:
Total measured at fair value	34,784		47,305
Recurring | Level I | Warrant
Financial Liabilities:
Convertible preferred stock warrant liability	0		0
Recurring | Level I | Corporate bonds
Financial Assets:
Short-term investments:	0		0
Recurring | Level I | Commercial paper
Financial Assets:
Short-term investments:	0		0
Recurring | Level I | US Government Debt Securities
Financial Assets:
Short-term investments:	0
Recurring | Level I | Money market funds
Financial Assets:
Cash equivalents:	34,784		47,305
Recurring | Level I | Commercial paper
Financial Assets:
Cash equivalents:	0		0
Recurring | Level II
Financial Assets:
Total measured at fair value	233,735		90,990
Recurring | Level II | Warrant
Financial Liabilities:
Convertible preferred stock warrant liability	0		0
Recurring | Level II | Corporate bonds
Financial Assets:
Short-term investments:	160,634		64,360
Recurring | Level II | Commercial paper
Financial Assets:
Short-term investments:	36,084		21,631
Recurring | Level II | US Government Debt Securities
Financial Assets:
Short-term investments:	13,976
Recurring | Level II | Money market funds
Financial Assets:
Cash equivalents:	0		0
Recurring | Level II | Commercial paper
Financial Assets:
Cash equivalents:	23,041		4,999
Recurring | Level III
Financial Assets:
Total measured at fair value	0		0
Recurring | Level III | Warrant
Financial Liabilities:
Convertible preferred stock warrant liability			9,679
Recurring | Level III | Corporate bonds
Financial Assets:
Short-term investments:	0		0
Recurring | Level III | Commercial paper
Financial Assets:
Short-term investments:	0		0
Recurring | Level III | US Government Debt Securities
Financial Assets:
Short-term investments:	0
Recurring | Level III | Money market funds
Financial Assets:
Cash equivalents:	0		0
Recurring | Level III | Commercial paper
Financial Assets:
Cash equivalents:	$ 0		$ 0