Fair Value of Financial Instruments - Contractual Maturity (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	$ 231,334
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	49,681
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	51,270
Debt Securities, Available-for-sale	332,285
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	12,299
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	0
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	0
US government agencies
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	187,883
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	13,527
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	14,730
Debt Securities, Available-for-sale	216,140
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	30,100
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	21,330
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	11,281
Debt Securities, Available-for-sale	62,711
Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	1,052
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	14,824
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	25,259
Debt Securities, Available-for-sale	$ 41,135