6. INVESTMENTS

Available-for-sale securities are carried at fair value, with the unrealized gains and losses reported in comprehensive loss. The amortized cost of debt securities in this category is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion is included in interest income. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in other income or expense. The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in interest income.

Investments consist of the following (in thousands):


	September 30, 2017		December 31, 2016
Certificates of deposit	$	240	$	960
Commercial paper		66,735		49,245
Corporate debt securities		363,012		204,436
Securities of government sponsored entities		23,471		12,932

Total investments	$	453,458	$	267,573

The following is a summary of investments classified as available-for-sale securities (in thousands):


	Contractual Maturity (in years)		Amortized Cost		Gross Unrealized Gains(1)		Gross Unrealized Losses(1)			Aggregate Estimated Fair Value
September 30, 2017:
Classified as current assets:
Certificates of deposit		Less than 1	$	240	$	—	$	—		$	240
Commercial paper		Less than 1		66,759		—		(24	)		66,735
Corporate debt securities		Less than 1		163,870		3		(131	)		163,742
Securities of government-sponsored entities		Less than 1		12,503		—		(21	)		12,482

Total short-term available-for-sale securities			$	243,372	$	3	$	(176	)	$	243,199

Classified as non-current assets:
Corporate debt securities		1 to 2	$	199,615	$	—	$	(345	)	$	199,270
Securities of government-sponsored entities		1 to 2		11,005		—		(16	)		10,989

Total long-term available-for-sale securities			$	210,620	$	—	$	(361	)	$	210,259

December 31, 2016:
Classified as current assets:
Certificates of deposit		Less than 1	$	960	$	—	$	—		$	960
Commercial paper		Less than 1		49,280		3		(38	)		49,245
Corporate debt securities		Less than 1		168,548		3		(117	)		168,434
Securities of government-sponsored entities		Less than 1		5,448		—		(4	)		5,444

Total short-term available-for-sale securities			$	224,236	$	6	$	(159	)	$	224,083

Classified as non-current assets:
Corporate debt securities		1 to 2	$	36,149	$	—	$	(147	)	$	36,002
Securities of government-sponsored entities		1 to 2		7,506		—		(18	)		7,488

Total long-term available-for-sale securities			$	43,655	$	—	$	(165	)	$	43,490

(1)	Unrealized gains and losses are included in other comprehensive loss.

The following table presents gross unrealized losses and fair value for those available-for-sale investments that were in an unrealized loss position as of September 30, 2017 and December 31, 2016, aggregated by investment category and length of time that individual securities have been in a continuous loss position (in thousands):


	Less Than 12 Months					12 Months or Greater					Total
	Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses
September 30, 2017:
Commercial paper	$	66,735	$	(24	)	$	—	$	—		$	66,735	$	(24	)
Corporate debt securities		325,223		(463	)		14,298		(13	)		339,521		(476	)
Securities of government-sponsored entities		15,988		(17	)		7,483		(20	)		23,471		(37	)

Total	$	407,946	$	(504	)	$	21,781	$	(33	)	$	429,727	$	(537	)

December 31, 2016:
Commercial paper	$	43,781	$	(38	)	$	—	$	—		$	43,781	$	(38	)
Corporate debt securities		185,243		(261	)		9,144		(3	)		194,387		(264	)
Securities of government-sponsored entities		12,932		(22	)		—		—			12,932		(22	)

Total	$	241,956	$	(321	)	$	9,144	$	(3	)	$	251,100	$	(324	)

The primary objective of the Company’s investment portfolio is to enhance overall returns in an efficient manner while maintaining safety of principal, prudent levels of liquidity and acceptable levels of risk. The Company’s investment policy limits interest-bearing security investments to certain types of instruments issued by institutions with primarily investment grade credit ratings and places restrictions on maturities and concentration by asset class and issuer.

The Company reviews the available-for-sale investments for other-than-temporary declines in fair value below cost basis each quarter and whenever events or changes in circumstances indicate that the cost basis of an asset may not be recoverable. This evaluation is based on a number of factors, including the length of time and the extent to which the fair value has been below the cost basis and adverse conditions related specifically to the security, including any changes to the credit rating of the security, and the intent to sell, or whether the Company will more likely than not be required to sell the security before recovery of its amortized cost basis. The assessment of whether a security is other-than-temporarily impaired could change in the future due to new developments or changes in assumptions related to any particular security. As of September 30, 2017 and December 31, 2016, the Company believed the cost bases for available-for-sale investments were recoverable in all material respects.