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Fair Value of Financial Instruments - Fair Value Hierarchy (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | $ 252,568 |
Level 1 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | $ 0
Level 2 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 252,568 | 216,435
Level 3 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | 0
Commercial paper |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 20,933 |
Commercial paper | Level 1 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | 0
Commercial paper | Level 2 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 20,933 | 28,378
Commercial paper | Level 3 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | 0
US government agencies |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 159,019 |
US government agencies | Level 1 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | 0
US government agencies | Level 2 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 159,019 | 157,048
US government agencies | Level 3 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | 0
Corporate bonds |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 69,269 |
Corporate bonds | Level 1 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | 0
Corporate bonds | Level 2 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 69,269 | 24,681
Corporate bonds | Level 3 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | 0
Asset backed securities |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 3,347 |
Asset backed securities | Level 1 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | 0
Asset backed securities | Level 2 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 3,347 | 6,328
Asset backed securities | Level 3 | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 0 | 0
Fair Value | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 252,568 | 216,435
Fair Value | Commercial paper | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 20,933 | 28,378
Fair Value | US government agencies | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 159,019 | 157,048
Fair Value | Corporate bonds | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | 69,269 | 24,681
Fair Value | Asset backed securities | Recurring Basis |  |
Assets, Fair Value Disclosure [Abstract] |  |
Available-for-sale securities, fair value | $ 3,347 | $ 6,328

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