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Cash Equivalents, Marketable Securities and Equity Securities (Details) $ in Thousands | 9 Months Ended |
--- | --- | ---
Sep. 30, 2018 USD ($) item | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($)
Cash Equivalents, Marketable Securities and Equity Securities |  |  |
Amortized Cost | $ 341,000 |  | $ 329,215
Gross Unrealized Losses | (82) |  | (76)
Fair Value | $ 340,918 |  | 329,139
Equity securities included in other non-current assets: |  |  |
Number of securities in an unrealized loss position | item | 41 |  |
Securities held by the Company in an unrealized loss position for less than 12 months | $ 230,300 |  |
Number of securities in an unrealized loss position for more than 12 months | item | 0 |  |
Realized gains (losses) on available-for-sale securities | $ 0 | $ 0 |
Government agency securities |  |  |
Cash Equivalents, Marketable Securities and Equity Securities |  |  |
Amortized Cost | 29,922 |  | 58,979
Gross Unrealized Losses | (15) |  | (29)
Fair Value | 29,907 |  | 58,950
Corporate equity securities |  |  |
Equity securities included in other non-current assets: |  |  |
Amortized Cost | 3,667 |  |
Fair Value | 3,667 |  |
Money market funds |  |  |
Cash Equivalents, Marketable Securities and Equity Securities |  |  |
Amortized Cost | 106,974 |  | 134,635
Fair Value | 106,974 |  | 134,635
U.S. Treasuries |  |  |
Cash Equivalents, Marketable Securities and Equity Securities |  |  |
Amortized Cost | 200,437 |  | 135,601
Gross Unrealized Losses | (67) |  | (47)
Fair Value | $ 200,370 |  | $ 135,554

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