Fair Value of Financial Instruments (Details 3) - USD ($) $ in Thousands	Jul. 31, 2019	Jul. 31, 2018
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Expected maturities for the year ending July 31, 2019	$ 1,046,378
Expected maturities for the year ending July 31, 2020	213,524
Estimated Fair Value	1,259,902	$ 1,184,381
U.S. Government agency securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Expected maturities for the year ending July 31, 2019	39,166
Expected maturities for the year ending July 31, 2020	16,713
Estimated Fair Value	55,879	8,973
Commercial paper
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Expected maturities for the year ending July 31, 2019	239,333
Expected maturities for the year ending July 31, 2020	0
Estimated Fair Value	239,333	471,829
Corporate bonds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Expected maturities for the year ending July 31, 2019	481,568
Expected maturities for the year ending July 31, 2020	186,020
Estimated Fair Value	667,588	440,852
U.S. Government bonds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Expected maturities for the year ending July 31, 2019	123,600
Expected maturities for the year ending July 31, 2020	6,995
Estimated Fair Value	130,595	89,931
Certificates of deposit
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Expected maturities for the year ending July 31, 2019	47,000
Expected maturities for the year ending July 31, 2020	3,796
Estimated Fair Value	50,796	82,030
Money market funds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Expected maturities for the year ending July 31, 2019	115,711
Expected maturities for the year ending July 31, 2020	0
Estimated Fair Value	$ 115,711	$ 90,766