Short-term Investments

3 Months Ended

Mar. 31, 2019

Investments, Debt and Equity Securities [Abstract]

Short-term Investments

Short-term Investments

The following tables summarize the Company's short-term investments (dollars in thousands):

		As of March 31, 2019
	Maturity	Amortized cost		Gross unrealized gains		Gross unrealized losses			Estimated fair value
Corporate debt securities	1 year or less	$	91,173	$	75	$	(4	)	$	91,244
Commercial paper	1 year or less	114,219		44		(1		)	114,262
U.S. Treasury bills	1 year or less	18,881		5		—			18,886
Other sovereign securities	1 year or less	1,988		—		—			1,988
		$	226,261	$	124	$	(5	)	$	226,380

		As of December 31, 2018
	Maturity	Amortized cost		Gross unrealized gains		Gross unrealized losses			Estimated fair value
Corporate debt securities	1 year or less	$	111,933	$	26	$	(43	)	$	111,916
Commercial paper	1 year or less	74,433		—		(24		)	74,409
U.S. Treasury bills	1 year or less	3,771		—		—			3,771
		$	190,137	$	26	$	(67	)	$	190,096

The Company has classified all of its investment securities as available-for-sale as the sale of such securities may be required prior to maturity to implement management strategies, and accordingly, carries these investments at fair value. Unrealized gains and losses on available-for-sale securities are included as a component of comprehensive loss. At March 31, 2019, the Company did not have any securities in material unrealized loss positions. The Company reviews its investments to identify and evaluate investments that have an indication of possible other-than-temporary impairment. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition and near-term prospects of the investee, and the Company’s intent and ability to hold the investment for a period of time sufficient to allow for any anticipated recovery in market value. The Company does not intend to sell any investments prior to recovery of their amortized cost basis for any investments in an unrealized loss position.