Fair Value of Financial Instruments - Cash and Cash Equivalents, Available-for-sale Securities Reconciliation (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019 |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized Gains | $ 1,966 | $ 1,071 |
Unrealized (Losses) | (556) | (5) |
Debt Securities, Available-for-sale, Amortized Cost | 433,319 | 417,332 |
Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 147,565 | 87,559 |
Fair Value | 434,729 | 418,398 |
amortized cost [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 147,554 | 87,559 |
Cash and cash equivalents: | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 11 | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0 |
Short-term investments | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Debt Securities, Available-for-sale, Amortized Cost | 183,308 | 211,089 |
Current Assets [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized Gains | 1,163 | 246 |
Unrealized (Losses) | (84) | (4) |
Current Assets [Member] | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Fair Value | 184,387 | 211,331 |
Long-term investments | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized Gains | 792 | 825 |
Unrealized (Losses) | (472) | (1) |
Debt Securities, Available-for-sale, Amortized Cost | | 118,684 |
Long-term investments | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Fair Value | 102,777 | 119,508 |
Long-term investments | Reported Value Measurement [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Debt Securities, Available-for-sale, Amortized Cost | 102,457 | |
Asset backed securities | Short-term investments | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized Gains | 9 | 11 |
Unrealized (Losses) | 0 | 0 |
Debt Securities, Available-for-sale, Amortized Cost | 3,017 | 2,438 |
Asset backed securities | Current Assets [Member] | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Fair Value | 3,026 | 2,449 |
Asset backed securities | Long-term investments | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized Gains | 79 | 193 |
Unrealized (Losses) | (72) | (1) |
Debt Securities, Available-for-sale, Amortized Cost | 39,983 | 40,001 |
Asset backed securities | Long-term investments | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Fair Value | 39,990 | 40,193 |
Corporate bonds | Cash and cash equivalents: | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 1 | |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0 |
Corporate bonds | Cash and cash equivalents: | amortized cost [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 1,517 | |
Corporate bonds | Short-term investments | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized Gains | 17 | 51 |
Unrealized (Losses) | (84) | (1) |
Debt Securities, Available-for-sale, Amortized Cost | 23,975 | 33,048 |
Corporate bonds | Current Assets [Member] | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Fair Value | 23,908 | 33,098 |
Corporate bonds | Long-term investments | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized Gains | 169 | 262 |
Unrealized (Losses) | (390) | 0 |
Debt Securities, Available-for-sale, Amortized Cost | 46,065 | 32,236 |
Corporate bonds | Long-term investments | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Fair Value | 45,844 | 32,498 |
US government agencies | Cash and cash equivalents: | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 10 | |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | |
US government agencies | Cash and cash equivalents: | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 15,449 | |
US government agencies | Cash and cash equivalents: | amortized cost [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 15,439 | |
US government agencies | Short-term investments | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized Gains | 1,137 | 184 |
Unrealized (Losses) | 0 | (3) |
Debt Securities, Available-for-sale, Amortized Cost | 155,316 | 173,364 |
US government agencies | Current Assets [Member] | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Fair Value | 156,453 | 173,545 |
US government agencies | Long-term investments | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized Gains | 544 | 370 |
Unrealized (Losses) | (10) | 0 |
Debt Securities, Available-for-sale, Amortized Cost | 16,409 | 46,447 |
US government agencies | Long-term investments | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Fair Value | 16,943 | 46,817 |
Commercial Paper [Member] | Cash and cash equivalents: | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | |
Commercial Paper [Member] | Cash and cash equivalents: | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 1,200 | 3,399 |
Commercial Paper [Member] | Cash and cash equivalents: | amortized cost [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 1,200 | 3,399 |
Commercial Paper [Member] | Short-term investments | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Debt Securities, Available-for-sale, Amortized Cost | 1,000 | 2,239 |
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | |
Commercial Paper [Member] | Current Assets [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Unrealized (Losses) | | 0 |
Commercial Paper [Member] | Current Assets [Member] | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Fair Value | 1,000 | 2,239 |
Cash [Member] | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 94,365 | |
Cash [Member] | Cash and cash equivalents: | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 94,365 | 84,102 |
Money market funds | Cash and cash equivalents: | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 35,033 | 58 |
Money market funds | Cash and cash equivalents: | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 35,033 | $ 58 |
Corporate bonds | Cash and cash equivalents: | Fair Value, Recurring [Member] | | |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | $ 1,518 | |