3. 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 investment 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 investment 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 investment income.

Investments consist of the following (in thousands):

	June 30, 2018		December 31, 2017
Commercial paper	$	37,157	$	75,362
Corporate debt securities		543,648		414,815
Securities of government sponsored entities		10,901		18,401
Total investments	$	591,706	$	508,578

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
June 30, 2018:
Classified as current assets:
Commercial paper	Less than 1	$	37,211	$	—	$	(54	)	$	37,157
Corporate debt securities	Less than 1		300,451		—		(1,243	)		299,208
Securities of government-sponsored entities	Less than 1		6,000		—		(47	)		5,953
Total short-term available-for-sale securities		$	343,662	$	—	$	(1,344	)	$	342,318
Classified as non-current assets:
Corporate debt securities	1 to 2	$	245,850	$	—	$	(1,410	)	$	244,440
Securities of government-sponsored entities	1 to 2		5,003		—		(55	)		4,948
Total long-term available-for-sale securities		$	250,853	$	—	$	(1,465	)	$	249,388
December 31, 2017:
Classified as current assets:
Commercial paper	Less than 1	$	75,396	$	1	$	(35	)	$	75,362
Corporate debt securities	Less than 1		178,776		—		(400	)		178,376
Securities of government-sponsored entities	Less than 1		7,503		—		(24	)		7,479
Total short-term available-for-sale securities		$	261,675	$	1	$	(459	)	$	261,217
Classified as non-current assets:
Corporate debt securities	1 to 2	$	237,749	$	—	$	(1,310	)	$	236,439
Securities of government-sponsored entities	1 to 2		11,004		—		(82	)		10,922
Total long-term available-for-sale securities		$	248,753	$	—	$	(1,392	)	$	247,361

(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 June 30, 2018 and December 31, 2017, 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
June 30, 2018:
Commercial paper	$	37,157	$	(54	)	$	—	$	—		$	37,157	$	(54	)
Corporate debt securities		398,814		(1,803	)		138,935		(850	)		537,749		(2,653	)
Securities of government-sponsored entities		10,901		(102	)		—		—			10,901		(102	)
Total	$	446,872	$	(1,959	)	$	138,935	$	(850	)	$	585,807	$	(2,809	)
December 31, 2017:
Commercial paper	$	62,602	$	(35	)	$	—	$	—		$	62,602	$	(35	)
Corporate debt securities		386,728		(1,660	)		28,087		(50	)		414,815		(1,710	)
Securities of government-sponsored entities		10,922		(82	)		7,479		(24	)		18,401		(106	)
Total	$	460,252	$	(1,777	)	$	35,566	$	(74	)	$	495,818	$	(1,851	)

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 June 30, 2018 and December 31, 2017, the Company believed the cost bases for available-for-sale investments were recoverable in all material respects.