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Fair Value of Financial Instruments - Cash and Cash Equivalents, Available-for-sale Securities Reconciliation (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
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Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | $ 343,204 | $ 289,739
Unrealized Gains | 43 | 7
Unrealized Losses | (162) | (165)
Fair Value | 343,085 | 289,581
Cash and cash equivalents |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 90,517 | 86,737
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
Fair Value | 90,517 | 86,737
Short-term investments |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 212,342 | 157,167
Unrealized Gains | 10 | 5
Unrealized Losses | (142) | (53)
Fair Value | 212,210 | 157,119
Long-term investments |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 40,345 | 45,835
Unrealized Gains | 33 | 2
Unrealized Losses | (20) | (112)
Fair Value | 40,358 | 45,725
Cash | Cash and cash equivalents |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 90,472 | 72,673
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
Fair Value | 90,472 | 72,673
Money market funds | Cash and cash equivalents |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 45 | 473
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
Fair Value | 45 | 473
Commercial paper | Cash and cash equivalents |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost |  | 13,591
Unrealized Gains |  | 0
Unrealized Losses |  | 0
Fair Value |  | 13,591
Commercial paper | Short-term investments |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 20,930 | 14,782
Unrealized Gains | 4 | 5
Unrealized Losses | (1) | 0
Fair Value | 20,933 | 14,787
Corporate bonds | Short-term investments |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 31,786 | 13,490
Unrealized Gains | 3 | 0
Unrealized Losses | (22) | (11)
Fair Value | 31,767 | 13,479
Corporate bonds | Long-term investments |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 37,490 | 11,243
Unrealized Gains | 32 | 0
Unrealized Losses | (20) | (41)
Fair Value | 37,502 | 11,202
Asset backed securities | Short-term investments |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 491 | 1,235
Unrealized Gains | 0 |
Unrealized Losses | 0 |
Fair Value | 491 | 1,235
Asset backed securities | Long-term investments |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 2,855 | 5,091
Unrealized Gains | 1 | 2
Unrealized Losses | 0 | 0
Fair Value | 2,856 | 5,093
US government agencies | Short-term investments |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost | 159,135 | 127,660
Unrealized Gains | 3 |
Unrealized Losses | (119) | (42)
Fair Value | $ 159,019 | 127,618
US government agencies | Long-term investments |  |
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] |  |
Amortized Cost |  | 29,501
Unrealized Gains |  | 0
Unrealized Losses |  | (71)
Fair Value |  | $ 29,430

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 + Details
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