| Marketable Securities  - Continuous Unrealized Loss Position (Details) $ in Thousands | Sep. 30, 2019  USD ($) | 
|---|---|
| Debt Securities, Available-for-sale [Line Items] | |
| Investments in continuous loss position, 12 more or more | $ 260,400 | 
| Fair Value | 431,792 | 
| Gross Unrealized Losses | (426) | 
| Corporate debt securities | |
| Debt Securities, Available-for-sale [Line Items] | |
| Fair Value | 182,963 | 
| Gross Unrealized Losses | (112) | 
| Sovereign securities | |
| Debt Securities, Available-for-sale [Line Items] | |
| Fair Value | 1,997 | 
| Gross Unrealized Losses | (3) | 
| U.S. Government agency securities | |
| Debt Securities, Available-for-sale [Line Items] | |
| Fair Value | 79,082 | 
| Gross Unrealized Losses | (72) | 
| U.S. Treasury securities | |
| Debt Securities, Available-for-sale [Line Items] | |
| Fair Value | 167,750 | 
| Gross Unrealized Losses | $ (239) | 
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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