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Summary of Significant Accounting Policies - Cash, Marketable Securities, and Property, Plant and Equipment (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Property, Plant and Equipment [Line Items] |  |
Unbilled Contracts Receivable | $ 7.4 | $ 23.3
Cash and Cash Equivalents |  |
Maximum time period for which an investment is considered a cash equivalent | 3 months |
Minimum |  |
Marketable Securities |  |
Maturity period of fixed income investments | 3 months |
Maturity period of marketable securities to be classified as long-term | 12 months |
Minimum | Property, Equipment, and Leasehold Improvements |  |
Property and Equipment |  |
Estimated useful life of property and equipment and leasehold improvements | 3 years |
Maximum | Property, Equipment, and Leasehold Improvements |  |
Property and Equipment |  |
Estimated useful life of property and equipment and leasehold improvements | 5 years |

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- Definition Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.3(c)(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 912 -SubTopic 310 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=68054760&loc=d3e55302-109406 + Details
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