6. Investments

The carrying value and amortized cost of the Company’s investments, summarized by major security type, consisted of the following (in thousands):

	June 30, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses			Estimated Fair Value
U.S. Treasury notes	$	—	$	—	$	—		$	—
Government sponsored enterprise securities		—		—		—			—
Corporate debt securities		139,304		—		(502	)		138,802
Commercial paper		43,879		6		(14	)		43,871
Equity securities		3,149		—		(247	)		2,902
	$	186,332	$	6	$	(763	)	$	185,575

	December 31, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses			Estimated Fair Value
U.S. Treasury notes	$	32,976	$	—	$	(12	)	$	32,964
Government sponsored enterprise securities		10,082		—		(10	)		10,072
Corporate debt securities		138,650		1		(321	)		138,330
Commercial paper		90,623		—		(65	)		90,558
	$	272,331	$	1	$	(408	)	$	271,924

The Company has classified all of its available-for-sale investment securities, including those with maturities beyond one year, as current assets on its consolidated balance sheets based on the highly liquid nature of the investment securities and because these investment securities are considered available for use in current operations. As of June 30, 2018 and December 31, 2017, the Company held $44.2 million and $48.7 million, respectively, of available-for-sale investment securities with contractual maturity dates of more than one year and less than two years. The Company has classified all equity securities as other assets on its consolidated balance sheets.

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:
U.S. Treasury notes	$	—	$	—		$	—	$	—	$	—	$	—
Government sponsored enterprise securities		—		—			—		—		—		—
Corporate debt securities		136,308		(502	)		—		—		136,308		(502	)
Commercial paper		24,061		(14	)		—		—		24,061		(14	)
Total	$	160,369	$	(516	)	$	—	$	—	$	160,369	$	(516	)
December 31, 2017:
U.S. Treasury notes	$	32,964	$	(12	)	$	—	$	—		32,964		(12	)
Government sponsored enterprise securities		10,072		(10	)		—		—		10,072		(10	)
Corporate debt securities		129,820		(321	)		—		—		129,820		(321	)
Commercial paper		90,558		(65	)		—		—		90,558		(65	)
Total	$	263,414	$	(408	)	$	—	$	—	$	263,414	$	(408	)

At each reporting date, the Company performs an evaluation of impairment to determine if any unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and the Company’s intent and ability to hold the investment until recovery of its amortized cost basis. The Company intends, and has the ability, to hold its investments in unrealized loss positions until their amortized cost basis has been recovered. Based on its evaluation, the Company determined that its unrealized losses were not other-than-temporary at June 30, 2018 and December 31, 2017.