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Fair Value Measurements Fair Value Measurements (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Assets, Fair Value Disclosure [Abstract] |  |
Money Market Funds | $ 327,869 | $ 360,746
US Government Securities, at Carrying Value | 50,147 | 0
Municipal bonds | 10,634 | 1,960
Total | 388,650 | 362,706
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract] |  |
Less than one year | 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 | 60,791 |
US Treasury Securities [Member] |  |
Debt Securities, Available-for-sale [Abstract] |  |
Available-for-sale Securities, Amortized Cost Basis | 50,150 | 0
Available-for-sale Securities, Gross Unrealized Gain | 0 | 0
Available-for-sale Securities, Gross Unrealized Loss | (3) | 0
Fair Value | 50,147 |
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract] |  |
Fair Value | 50,147 |
Municipal Bond [Member] |  |
Debt Securities, Available-for-sale [Abstract] |  |
Available-for-sale Securities, Amortized Cost Basis | 10,641 | 1,965
Available-for-sale Securities, Gross Unrealized Gain | 0 | 0
Available-for-sale Securities, Gross Unrealized Loss | (7) | (5)
Fair Value | 10,634 | 1,960
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract] |  |
Fair Value | 10,634 | 1,960
Level 1 [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Money Market Funds | 327,869 | 360,746
US Government Securities, at Carrying Value | 0 | 0
Municipal bonds | 0 | 0
Total | 327,869 | 360,746
Level 2 [Member] |  |
Assets, Fair Value Disclosure [Abstract] |  |
Money Market Funds | 0 | 0
US Government Securities, at Carrying Value | 50,147 | 0
Municipal bonds | 10,634 | 1,960
Total | $ 60,781 | $ 1,960

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + 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 Amortized cost of investment in debt security measured at fair value with change recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next 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 (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 + Details
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- Definition Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in next 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 (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803 + 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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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Debt (bills, notes or bonds) that are issued by the government of the United States which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months. + References No definition available. + Details
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