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Fair Value Measurements Fair Value Measurements (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Money market funds | $ 232,777 | $ 325,819
US Government Securities, at Carrying Value | 61,529 | 52,197
Municipal bonds | 0 | 10,634
Total | 294,306 | 388,650
Available-for-sale Securities, Amortized Cost Basis | 60,854 | 60,790
Available-for-sale Securities, Gross Unrealized Gain | 27 | 1
Available-for-sale Securities, Gross Unrealized Loss | (1) | (10)
Available-for-sale Securities | 60,880 | 60,781
Debt Securities, Available-for-sale, Amortized Cost, Fiscal Year Maturity [Abstract] |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost | 60,854 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost | 0 |
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract] |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value | 60,880 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value | 0 |
US Treasury Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 60,854 | 50,150
Available-for-sale Securities, Gross Unrealized Gain | 27 | 0
Available-for-sale Securities, Gross Unrealized Loss | (1) | (3)
Available-for-sale Securities | 60,880 | 50,147
Municipal Bond [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 0 | 10,640
Available-for-sale Securities, Gross Unrealized Gain | 0 | 1
Available-for-sale Securities, Gross Unrealized Loss | 0 | (7)
Available-for-sale Securities | 0 | 10,634
Fair Value, Inputs, Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Money market funds | 232,777 | 325,819
US Government Securities, at Carrying Value | 61,529 | 52,197
Municipal bonds | 0 | 0
Total | 294,306 | 378,016
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Money market funds | 0 | 0
US Government Securities, at Carrying Value | 0 | 0
Municipal bonds | 0 | 10,634
Total | $ 0 | $ 10,634

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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 + Details
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- Definition Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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