Short-term and Long-term Investments

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Short-term and Long-term Investments

Short-term and Long-term Investments

The Company's investments are classified as available-for-sale. The following table summarizes the amortized cost, gross unrealized gains and losses and fair value of available-for-sale investments as of December 31, 2017:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Aggregate Fair Value
U.S. government debt securities	$	6,000	$	—	$	(2	)	$	5,998
Certificates of deposit	275		—		—			275
Total	$	6,275	$	—	$	(2	)	$	6,273

The following table summarizes the amortized cost, gross unrealized gains and losses and fair value of available-for-sale investments as of December 31, 2016:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Aggregate Fair Value
U.S. government debt securities	$	180,412	$	5	$	(94	)	$	180,323
Certificates of deposit	272		—		—			272
Total	$	180,684	$	5	$	(94	)	$	180,595

For more information on the Company's method for determining the fair value of its assets, see Note 2 – "Fair Value of Financial Instruments".

As of December 31, 2017, all of the available-for-sale investments were due within one year based on their contractual maturities.

Changes in market interest rates and bond yields cause certain investments to fall below their cost basis, resulting in unrealized losses on investments. The unrealized losses of the Company's investments were primarily a result of unfavorable changes in interest rates subsequent to the initial purchase of these investments and are not significant as of December 31, 2017.

As of December 31, 2017 and 2016, the Company did not consider any of its investments to be other-than-temporarily impaired. When evaluating its investments for other-than-temporary impairment, the Company reviews 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, the Company's ability and intent to hold the security and whether it is more likely than not that it will be required to sell the investment before recovery of its cost basis.