(6) INVESTMENTS

The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s available-for-sale debt securities by major security type at December 31, 2017 and 2016 are summarized in the tables below:

	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value at December 31, 2017
Corporate debt securities	$	707,652	$	150	$	(2,553	)	$	705,249
Commercial paper		24,566		—		—			24,566
U.S. government agency securities		472,593		—		(1,975	)		470,618
Foreign and other		25,540		150		(64	)		25,626
Total	$	1,230,351	$	300	$	(4,592	)	$	1,226,059

	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		641,670		329		(2,282	)		639,717
Commercial paper		16,075		—		—			16,075
U.S. government agency securities		310,635		37		(747	)		309,925
Foreign and other		48		86		—			134
Total	$	971,228	$	452	$	(3,029	)	$	968,651

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

	December 31,
	2017		2016
Maturing in one year or less	$	840,274	$	395,940
Maturing after one year through five years		385,785		572,711
Total	$	1,226,059	$	968,651

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, 2017, 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 was 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.