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Cash and Cash Equivalents and Investments (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Cash and Cash Equivalents |  |  |
Realized Investment Gains (Losses) | $ 0 | $ 7,000 | $ 0
Cash |  |  |
Cash and Cash Equivalents |  |  |
Available-for-sale Securities, Amortized Cost Basis | 80,386,000 | 61,661,000 |
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments | 0 | 0 |
Available-for-sale Securities, Gross Unrealized Losses | 0 | 0 |
Available-for-sale Securities, Current | 80,386,000 | 61,661,000 |
US Treasury Securities |  |  |
Cash and Cash Equivalents |  |  |
Available-for-sale Securities, Amortized Cost Basis | 73,983,000 | 222,316,000 |
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments | 0 | 0 |
Available-for-sale Securities, Gross Unrealized Losses | (9,000) | (168,000) |
Available-for-sale Securities, Current | 73,974,000 | 222,148,000 |
Corporate Debt Securities [Member] |  |  |
Cash and Cash Equivalents |  |  |
Available-for-sale Securities, Amortized Cost Basis | 5,695,000 | 27,033,000 |
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments | 0 | 0 |
Available-for-sale Securities, Gross Unrealized Losses | (3,000) | (54,000) |
Available-for-sale Securities, Current | 5,692,000 | 26,979,000 |
Short-term Investments |  |  |
Cash and Cash Equivalents |  |  |
Available-for-sale Securities, Amortized Cost Basis | 79,678,000 | 249,349,000 |
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments | 0 | 0 |
Available-for-sale Securities, Gross Unrealized Losses | (12,000) | (222,000) |
Available-for-sale Securities, Current | 79,666,000 | 249,127,000 |
Cash and Cash Equivalents and Investments |  |  |
Cash and Cash Equivalents |  |  |
Available-for-sale Securities, Amortized Cost Basis | 160,064,000 | 311,010,000 |
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments | 0 | 0 |
Available-for-sale Securities, Gross Unrealized Losses | (12,000) | (222,000) |
Available-for-sale Securities, Current | $ 160,052,000 | $ 310,788,000 |

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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://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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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References No definition available. + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263 + Details
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