Fair Value of Financial Instruments (Details) - USD ($) | 9 Months Ended | |
Sep. 30, 2016 | Dec. 31, 2015 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount | $ 0 | |
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount | 0 | |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 | 0 | |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | 0 | |
Assets, Fair Value Disclosure [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 41,100,000 | $ 20,572,000 |
AFS Securities, Fair Value Disclosure | 80,509,000 | 96,202,000 |
Fair Value, Measurements, Recurring | | |
Assets, Fair Value Disclosure [Abstract] | | |
Total assets measured at fair value | 105,625,000 | 104,886,000 |
Fair Value, Measurements, Recurring | Level 1 | | |
Assets, Fair Value Disclosure [Abstract] | | |
Total assets measured at fair value | 18,266,000 | 8,684,000 |
Fair Value, Measurements, Recurring | Level 2 | | |
Assets, Fair Value Disclosure [Abstract] | | |
Total assets measured at fair value | 87,359,000 | 96,202,000 |
Fair Value, Measurements, Recurring | Money market funds | | |
Assets, Fair Value Disclosure [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 18,266,000 | 7,532,000 |
Fair Value, Measurements, Recurring | Money market funds | Level 1 | | |
Assets, Fair Value Disclosure [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 18,266,000 | 7,532,000 |
Fair Value, Measurements, Recurring | Money market funds | Level 2 | | |
Assets, Fair Value Disclosure [Abstract] | | |
Cash and Cash Equivalents, Fair Value Disclosure | 0 | 0 |
Fair Value, Measurements, Recurring | Commercial paper | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | 3,125,000 | 0 |
Fair Value, Measurements, Recurring | Commercial paper | Level 1 | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | | 0 |
Fair Value, Measurements, Recurring | Commercial paper | Level 2 | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | 3,125,000 | 0 |
Fair Value, Measurements, Recurring | U.S. government agency securities | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | 9,850,000 | 13,009,000 |
Fair Value, Measurements, Recurring | U.S. government agency securities | Level 1 | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | | 0 |
Fair Value, Measurements, Recurring | U.S. government agency securities | Level 2 | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | 9,850,000 | 13,009,000 |
Fair Value, Measurements, Recurring | U.S. Treasury securities | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | 5,404,000 | 5,843,000 |
Fair Value, Measurements, Recurring | U.S. Treasury securities | Level 1 | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | 0 | 0 |
Fair Value, Measurements, Recurring | U.S. Treasury securities | Level 2 | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | 5,404,000 | 5,843,000 |
Fair Value, Measurements, Recurring | Corporate debt securities | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | 68,980,000 | 78,502,000 |
Fair Value, Measurements, Recurring | Corporate debt securities | Level 1 | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | | 1,152,000 |
Fair Value, Measurements, Recurring | Corporate debt securities | Level 2 | | |
Assets, Fair Value Disclosure [Abstract] | | |
AFS Securities, Fair Value Disclosure | $ 68,980,000 | $ 77,350,000 |