Marketable Securities and Fair Value Measurements - Narrative (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Original maturity of highly liquid investments included in Cash and cash equivalents	40 months
Weighted average maturity	6 months	7 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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