Marketable Securities - Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands |
Sep. 30, 2020 |
Sep. 30, 2019 |
---|---|---|
Schedule of Available-for-sale Securities [Line Items] | ||
Amortized Cost | $ 330,923 | $ 348,876 |
Gross Unrealized Gains | 1,243 | 230 |
Gross Unrealized Losses | (14) | (87) |
Marketable securities | 332,152 | 349,019 |
Corporate Bonds [Member] | ||
Schedule of Available-for-sale Securities [Line Items] | ||
Amortized Cost | 143,274 | 111,690 |
Gross Unrealized Gains | 775 | 170 |
Gross Unrealized Losses | (14) | (23) |
Marketable securities | 144,035 | 111,837 |
U.S. Treasury Notes [Member] | ||
Schedule of Available-for-sale Securities [Line Items] | ||
Amortized Cost | 93,740 | 170,519 |
Gross Unrealized Gains | 468 | 60 |
Gross Unrealized Losses | (64) | |
Marketable securities | 94,208 | 170,515 |
Commercial Paper [Member] | ||
Schedule of Available-for-sale Securities [Line Items] | ||
Amortized Cost | 93,909 | 66,667 |
Marketable securities | $ 93,909 | $ 66,667 |