Fair Value Measurements - Summary of Financial Assets Measured and Recorded at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | $ 49,188 | $ 139,045 |
The Notes, Due 2023 [Member] | Convertible Debt [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Interest rate | 1.25% | |
Convertible debt, fair value | $ 278,700 | |
Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 234,497 | 245,826 |
Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 185,309 | 143,760 |
Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 49,188 | 102,066 |
Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 0 | 0 |
Corporate Bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 23,579 | 60,878 |
Corporate Bonds [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 23,579 | 60,878 |
Corporate Bonds [Member] | Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Corporate Bonds [Member] | Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 23,579 | 60,878 |
Corporate Bonds [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
U.S. Government Agencies [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | | 36,979 |
U.S. Government Agencies [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | | 36,979 |
U.S. Government Agencies [Member] | Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | | 36,979 |
U.S. Government Agencies [Member] | Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | | 0 |
U.S. Government Agencies [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | | 0 |
Commercial Paper [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 20,009 | 19,966 |
Commercial Paper [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 20,009 | 19,966 |
Commercial Paper [Member] | Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Commercial Paper [Member] | Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 20,009 | 19,966 |
Commercial Paper [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Agency Bonds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 3,101 | 12,242 |
Agency Bonds [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 3,101 | 12,242 |
Agency Bonds [Member] | Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Agency Bonds [Member] | Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 3,101 | 12,242 |
Agency Bonds [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Asset-backed Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 2,499 | 8,980 |
Asset-backed Securities [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 2,499 | 8,980 |
Asset-backed Securities [Member] | Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Asset-backed Securities [Member] | Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 2,499 | 8,980 |
Asset-backed Securities [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale securities | 0 | 0 |
Money Market Funds [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | 185,309 | 106,781 |
Money Market Funds [Member] | Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | 185,309 | 106,781 |
Money Market Funds [Member] | Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | 0 | 0 |
Money Market Funds [Member] | Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Money market funds | $ 0 | $ 0 |