Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2018

Financial assets and liabilities measured at fair value on a recurring basis

	Fair Value Measurements at March 31, 2018 Using
Description	Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Assets:
Cash equivalents:
Money market funds	$	111,866	$	—	$	—	$	111,866
Corporate debt securities	—		9,419		—		9,419
Government debt securities	24,997		—		—		24,997
Total cash equivalents	$	136,863	$	9,419	$	—	$	146,282

Short-term investments:
Government debt securities	$	65,239	$	219,493	$	—	$	284,732
Corporate debt securities	—		133,135		—		133,135
Total short-term investments	$	65,239	$	352,628	$	—	$	417,867

Other assets, net:
Auction rate securities	$	—	$	—	$	5,609	$	5,609
Total	$	—	$	—	$	5,609	$	5,609

Total	$	202,102	$	362,047	$	5,609	$	569,758

	Fair Value Measurements at December 30, 2017 Using
Description	Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Assets:
Cash equivalents:
Money market funds	$	106,047	$	—	$	—	$	106,047
Corporate debt securities	—		11,231		—		11,231
Government debt securities	53,615		1,453		—		55,068
Total cash equivalents	$	159,662	$	12,684	$	—	$	172,346

Short-term investments:
Government debt securities	$	94,575	$	228,247	$	—	$	322,822
Corporate debt securities	—		171,835		—		171,835
Total short-term investments	$	94,575	$	400,082	$	—	$	494,657

Other assets, net:
Auction rate securities	$	—	$	—	$	5,681	$	5,681
Total	$	—	$	—	$	5,681	$	5,681

Total	$	254,237	$	412,766	$	5,681	$	672,684

Summarization of contractual underlying maturities of available-for-sale investments

The following summarizes the contractual underlying maturities of the Company’s available-for-sale investments at March 31, 2018 (in thousands):

	Cost		Fair Value
Due in one year or less	$	338,056	$	337,433
Due after one year through ten years	135,923		134,453
Due after ten years	98,269		97,872
	$	572,248	$	569,758

Schedule of available-for-sale investments in continuous unrealized loss position by length of time

The available-for-sale investments that were in a continuous unrealized loss position, aggregated by length of time that individual securities have been in a continuous loss position, were as follows (in thousands):

	Less Than 12 Months					12 Months or Greater					Total
As of March 31, 2018	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Government debt securities	$	182,320	$	(1,059	)	$	4,432	$	(10	)	$	186,752	$	(1,069	)
Corporate debt securities	120,303		(929		)	11,327		(100		)	131,630		(1,029		)
Auction rate securities	—		—			5,609		(391		)	5,609		(391		)
	$	302,623	$	(1,988	)	$	21,368	$	(501	)	$	323,991	$	(2,489	)

	Less Than 12 Months					12 Months or Greater					Total
As of December 30, 2017	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Government debt securities	$	244,880	$	(931	)	$	3,027	$	(15	)	$	247,907	$	(946	)
Corporate debt securities	151,149		(447		)	11,578		(73		)	162,727		(520		)
Auction rate securities	—		—			5,681		(319		)	5,681		(319		)
	$	396,029	$	(1,378	)	$	20,286	$	(407	)	$	416,315	$	(1,785	)

Summary of quantitative information about level 3 asset fair value measurements

Auction rate securities

Fair Value at March 31, 2018 (000s)		Valuation Technique	Unobservable Input	Weighted Average
$	5,609	Discounted cash flow	Estimated yield	2.22%

			Expected holding period	10 years

			Estimated discount rate	3.63%

Summary of activity in Level 3 financial instruments

The following summarizes the activity in Level 3 financial instruments for the three months ended March 31, 2018 (in thousands):

Assets

Auction Rate Securities	Three Months Ended
Beginning balance	$	5,681
Loss included in other comprehensive loss	(72		)
Balance at March 31, 2018	$	5,609