NOTE 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 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 investment income.

Investments at December 31, 2017 and 2016 consisted of the following (in thousands):


	Years Ended December 31,
	2017		2016
Certificates of deposit	$	—	$	960
Commercial paper		75,362		49,245
Corporate debt securities		414,815		204,436
Securities of government-sponsored entities		18,401		12,932

Total investments	$	508,578	$	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
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

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 December 31, 2017 and 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
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	)

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	)