FAIR VALUE MEASUREMENTS - Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Apr. 30, 2018	Jul. 31, 2017
Financial Assets:
Short-term investments:	$ 546,675	$ 210,694	[1]
Recurring
Financial Assets:
Total measured at fair value	754,186	268,519
Cash	169,278	80,534
Total cash, cash equivalents and short-term investments	923,464	349,053
Recurring | Commitments
Financial Liabilities:
Convertible preferred stock warrant liability	924	4,295
Recurring | Corporate bonds
Financial Assets:
Short-term investments:	395,107	160,634
Recurring | Commercial paper
Financial Assets:
Short-term investments:	118,939	36,084
Recurring | U.S. government securities
Financial Assets:
Short-term investments:	32,629	13,976
Recurring | Money market funds
Financial Assets:
Cash equivalents:	95,258	34,784
Recurring | Commercial paper
Financial Assets:
Cash equivalents:	108,253	23,041
Recurring | Corporate bonds
Financial Assets:
Cash equivalents:	4,000
Recurring | Level I
Financial Assets:
Total measured at fair value	95,258	34,784
Recurring | Level I | Commitments
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 | U.S. government securities
Financial Assets:
Short-term investments:	0	0
Recurring | Level I | Money market funds
Financial Assets:
Cash equivalents:	95,258	34,784
Recurring | Level I | Commercial paper
Financial Assets:
Cash equivalents:		0
Recurring | Level I | Corporate bonds
Financial Assets:
Cash equivalents:	0
Recurring | Level II
Financial Assets:
Total measured at fair value	658,928	233,735
Recurring | Level II | Commitments
Financial Liabilities:
Convertible preferred stock warrant liability	0	0
Recurring | Level II | Corporate bonds
Financial Assets:
Short-term investments:	395,107	160,634
Recurring | Level II | Commercial paper
Financial Assets:
Short-term investments:	118,939	36,084
Recurring | Level II | U.S. government securities
Financial Assets:
Short-term investments:	32,629	13,976
Recurring | Level II | Money market funds
Financial Assets:
Cash equivalents:	0	0
Recurring | Level II | Commercial paper
Financial Assets:
Cash equivalents:	108,253	23,041
Recurring | Level II | Corporate bonds
Financial Assets:
Cash equivalents:	4,000
Recurring | Level III
Financial Assets:
Total measured at fair value	0	0
Recurring | Level III | Commitments
Financial Liabilities:
Convertible preferred stock warrant liability	924	4,295
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 | U.S. government 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
Recurring | Level III | Corporate bonds
Financial Assets:
Cash equivalents:	$ 0

[1] See Note 3 for a summary of adjustments related to the adoption of the new revenue recognition standard.