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Marketable Securities (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 81,750,000 |
Gross Unrealized Gains | 6,000 |
Gross Unrealized Losses | (6,000) |
Fair Value | 81,750,000 |
Other-than-temporary unrealized losses | 0 | $ 0
U.S. Treasury securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 0 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Fair Value | 0 |
Government-sponsored enterprises |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 17,389,000 |
Gross Unrealized Gains | 1,000 |
Gross Unrealized Losses | 0 |
Fair Value | 17,390,000 |
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 64,361,000 | 12,738,000
Gross Unrealized Gains | 5,000 | 0
Gross Unrealized Losses | (6,000) | (3,000)
Fair Value | $ 64,360,000 | $ 12,735,000

X
- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (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 (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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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). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References No definition available. + Details
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