(6) INVESTMENTS

All investments were classified as available-for-sale at December 31, 2016 and 2015. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s available-for-sale securities by major security type at December 31, 2016 and 2015 are summarized in the tables below:

	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value at December 31, 2016
Certificates of deposit	$	2,800	$	—	$	—		$	2,800
Corporate debt securities		633,072		329		(2,277	)		631,124
Commercial paper		16,075		—		—			16,075
U.S. government agency securities		304,635		37		(747	)		303,925
Greek government-issued bonds		48		86		—			134
Total	$	956,630	$	452	$	(3,024	)	$	954,058

	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value at December 31, 2015
Certificates of deposit	$	63,919	$	1	$	—		$	63,920
Corporate debt securities		358,625		20		(732	)		357,913
Commercial paper		12,733		—		—			12,733
U.S. government agency securities		186,882		—		(344	)		186,538
Greek government-issued bonds		48		79		—			127
Total	$	622,207	$	100	$	(1,076	)	$	621,231

As of December 31, 2016, the Company had one investment in marketable equity securities measured using quoted prices in its active market that is considered a strategic investment. During 2016, shares of strategic investments were sold for net realized losses of $0.1 million. As of December 31, 2016, the fair value of the Company’s strategic investment of $4.1 million included an unrealized gain of $2.3 million. As of December 31, 2015, the fair value of the Company’s strategic investments of $18.1 million included an unrealized gain of $12.7 million. Strategic investments are recorded in Other Assets in the Company’s Consolidated Balance Sheets.

The fair values of available-for-sale securities by contractual maturity were as follows:

	December 31,
	2016		2015
Maturing in one year or less	$	381,347	$	195,579
Maturing after one year through five years		572,711		425,652
Total	$	954,058	$	621,231

Impairment assessments are made at the individual security level each reporting period. When the fair value of an investment is less than its cost at the balance sheet date, a determination is made as to whether the impairment is other-than-temporary and, if it is other-than-temporary, an impairment loss is recognized in earnings equal to the difference between the investment’s amortized cost and fair value at such date. As of December 31, 2016, some of the Company’s investments were in an unrealized loss position. However, the Company has the ability and intent to hold all investments that have been in a continuous loss position until maturity or recovery, thus no other-than-temporary impairment is deemed to have occurred.

See Note 12 to these Consolidated Financial Statements for additional discussion regarding the fair value of the Company’s available-for-sale securities.