5)	Investments

Investments classified as short-term consist of the following:


	Years Ended December 31,
	2018		2017
Available-for-sale investments:
Time deposits and certificates of deposit	$	102	$	9,757
Bankers’ acceptance drafts		989		5,330
Asset-backed securities		9,113		36,990
Commercial paper		19,359		13,750
Corporate obligations		9,352		77,821
Municipal bonds		—		1,970
U.S. treasury obligations		13,298		28,078
U.S. agency obligations		21,613		35,738

	$	73,826	$	209,434

Investments classified as long-term consist of the following:


	Years Ended December 31,
	2018		2017
Available-for-sale investments:
Group insurance contracts	$	5,890	$	6,255
Cost method investments:
Minority interest in a private company		4,400		4,400

	$	10,290	$	10,655

The following table shows the gross unrealized gains and (losses) aggregated by investment category for available-for-sale investments:


As of December 31, 2018:	Cost		Gross Unrealized Gains		Gross Unrealized (Losses)			Estimated Fair Value
Short-term investments:
Available-for-sale investments:
Time deposits and certificates of deposit	$	102	$	—	$	—		$	102
Bankers’ acceptance drafts		989		—		—			989
Asset-backed securities		9,121		1		(9	)		9,113
Commercial paper		19,504		—		(145	)		19,359
Corporate obligations		9,367		—		(15	)		9,352
U.S. treasury obligations		13,294		4		—			13,298
U.S. agency obligations		21,617		2		(6	)		21,613

	$	73,994	$	7	$	(175	)	$	73,826


As of December 31, 2018:	Cost		Gross Unrealized Gains		Gross Unrealized (Losses)		Estimated Fair Value
Long-term investments:
Available-for-sale investments:
Group insurance contracts	$	5,546	$	344	$	—	$	5,890


As of December 31, 2017:	Cost		Gross Unrealized Gains		Gross Unrealized (Losses)			Estimated Fair Value
Short-term investments:
Available-for-sale investments:
Time deposits and certificates of deposit	$	9,756	$	1	$	—		$	9,757
Bankers’ acceptance drafts		5,330		—		—			5,330
Asset-backed securities		37,017		15		(42	)		36,990
Commercial paper		13,810		—		(60	)		13,750
Corporate obligations		77,788		58		(25	)		77,821
Municipal bonds		1,970		—		—			1,970
U.S. treasury obligations		28,054		24		—			28,078
U.S. agency obligations		35,728		10		—			35,738

	$	209,453	$	108	$	(127	)	$	209,434


As of December 31, 2017:	Cost		Gross Unrealized Gains		Gross Unrealized (Losses)		Estimated Fair Value
Long-term investments:
Available-for-sale investments:
Group insurance contracts	$	6,006	$	249	$	—	$	6,255

The tables above, which show the gross unrealized gains and (losses) aggregated by investment category for available-for-sale investments as of December 31, 2018 and 2017, reflect the inclusion within short-term investments of investments with contractual maturities greater than one year from the date of purchase. Management has the ability, if necessary, to liquidate any of its investments in order to meet the Company’s liquidity needs in the next 12 months. Accordingly, those investments with contractual maturities greater than one year from the date of purchase are classified as short-term on the accompanying balance sheets.

Interest income is accrued as earned. Dividend income is recognized as income on the date the stock trades “ex-dividend.” The cost of marketable securities sold is determined by the specific identification method and realized gains or losses are reflected in income and were not material in 2018, 2017 and 2016.