Marketable Securities and Fair Value Measurements Additional Information (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Original maturity of highly liquid investments included in cash and cash equivalents	40 months
Weighted average maturity	5 months	6 months

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.

No definition available.

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