Short-term Investments (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Amortized cost	$ 154,397	$ 43,176
Available-for-sale, Gross unrealized gains	14	0
Available-for-sale, Gross unrealized losses	(190)	(42)
Available-for-sale Securities	$ 154,221	$ 43,134
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Maximum Maturity Period	1 year	1 year
Available-for-sale, Amortized cost	$ 89,084	$ 24,264
Available-for-sale, Gross unrealized gains	0	0
Available-for-sale, Gross unrealized losses	(163)	(23)
Available-for-sale Securities	88,921	$ 24,241
Available-for-sale, Estimated fair value	$ 1,700
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Maximum Maturity Period	1 year	1 year
Available-for-sale, Amortized cost	$ 65,313	$ 18,912
Available-for-sale, Gross unrealized gains	14	0
Available-for-sale, Gross unrealized losses	(27)	(19)
Available-for-sale Securities	$ 65,300	$ 18,893

- Definition

Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

+ References + Details