Investments (Tables)

12 Months Ended

Dec. 31, 2017

Investments [Abstract]

Summary investment holdings

The following table summarizes the amortized cost, gross unrealized gains and losses, and fair value of our investments as measured using Level 2 inputs as of December 31, 2016 (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Costs		Gains		Losses		Value
U.S. Treasury bills	$	28,119	$	116	$	27	$	28,208
Corporate bonds	16,222		81		8		16,295
Total investments	$	44,341	$	197	$	35	$	44,503

Investments classified by contractual maturity date

The following table summarizes the amortized cost and fair value of our investments by contractual maturities as of December 31, 2016 (in thousands):

	Amortized		Fair
	Costs		Value
Due in one year or less	$	44,341	$	44,503

Schedule of unrealized loss on held-to-maturity securities

The following table summarizes our held-to-maturity securities that had been in a continuous unrealized loss position for less than twelve months as of December 31, 2016 (in thousands, except number of securities):

	Number of	Fair		Unrealized
	Securities	Value		Losses
U.S. Treasury bills	1	$	4,002	$	1