Short-Term Investments

12 Months Ended

Dec. 31, 2017

Cash And Cash Equivalents [Abstract]

Short-Term Investments

2. Short-Term Investments

The following tables summarize our amortized cost, gross unrealized gains and losses and fair value of our short-term investments as of December 31, 2017 (in thousands):

	December 31, 2017
			Gross		Gross
	Amortized		Unrealized		Unrealized			Aggregate
	Cost		Gains		Losses			Fair Value
Corporate debt securities	$	308,684	$	—	$	(178	)	$	308,506
Total available-for-sale short-term investments	$	308,684	$	—	$	(178	)	$	308,506

We had no available-for-sale short-term investments securities as of December 31, 2016.

All of our available-for-sale short-term investments have contractual maturities of one year or less as of December 31, 2017.

Changes in market interest rates and bond yields cause our short-term investments to fall below their cost basis, resulting in unrealized losses. As of December 31, 2017, we had unrealized losses of $0.2 million related to short-term investments that had a fair value of $143.0 million. At December 31, 2017, none of these investments were in a material continuous unrealized loss position for more than 12 months.

 As of December 31, 2017, we did not consider any of our short-term investments to be other-than-temporarily impaired. We do not intend to sell, nor do we believe it is more likely than not that we will be required to sell, any of the securities in an unrealized loss position. When evaluating our investments for other-than-temporary impairment, we review factors such as the length of time and extent to which fair value has been below its cost basis, the financial condition of the issuer, our ability and intent to hold the security to maturity and whether it is more likely than not that we will be required to sell the investment before recovery of the amortized cost basis.