Cash and Investments (Tables)

12 Months Ended

Dec. 31, 2015

Cash and Cash Equivalents [Abstract]

Schedule of cash, cash equivalents and marketable securities

The following table presents cash, cash equivalents, and available-for-sale investments for the periods presented (in thousands):

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Market Value
December 31, 2015:
Cash	$	180,790	$	—	$	—		$	180,790
Cash equivalents:
Money market funds	276,898		—		—			276,898
Commercial paper	12,048		1		—			12,049
US treasury securities	24,999		1		—			25,000
US agency securities	51,500		—		—			51,500
Marketable securities:
Commercial paper	55,350		47		(23		)	55,374
US treasury securities	874,968		20		(959		)	874,029
US agency securities	606,924		17		(739		)	606,202
Corporate debt securities	1,029,463		96		(2,661		)	1,026,898
Municipal securities	10,636		6		—			10,642
Total cash, cash equivalents, and marketable securities	$	3,123,576	$	188	$	(4,382	)	$	3,119,382
December 31, 2014:
Cash	$	213,724	$	—	$	—		$	213,724
Cash equivalents:
Money market funds	95,470		—		—			95,470
Commercial paper	54,340		4		—			54,344
US treasury securities	54,349		—		—			54,349
US agency securities	42,999		1		—			43,000
Marketable securities:
Commercial paper	122,345		33		(7		)	122,371
Certificates of deposit	5,925		2		—			5,927
US treasury securities	1,234,870		64		(366		)	1,234,568
US agency securities	881,843		393		(274		)	881,962
Corporate debt securities	723,412		225		(932		)	722,705
Municipal securities	14,893		4		(8		)	14,889
Total cash, cash equivalents, and marketable securities	$	3,444,170	$	726	$	(1,587	)	$	3,443,309

Schedule of unrealized losses on investments in continuous unrealized loss position

The following table presents unrealized losses on investments by investment category and length of time the investment has been in a continuous unrealized loss position as of the periods presented (in thousands):

	Less than 12 months					12 Months or Greater				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
December 31, 2015:
Commercial paper	$	28,482	$	(23	)	$	—	$	—	$	28,482	$	(23	)
US treasury securities	777,237		(959		)	—		—		777,237		(959		)
US agency securities	472,236		(739		)	—		—		472,236		(739		)
Corporate debt securities	943,276		(2,661		)	—		—		943,276		(2,661		)
Total	$	2,221,231	$	(4,382	)	$	—	$	—	$	2,221,231	$	(4,382	)
December 31, 2014:
Commercial paper	$	41,285	$	(7	)	$	—	$	—	$	41,285	$	(7	)
US treasury securities	1,019,771		(366		)	—		—		1,019,771		(366		)
US agency securities	468,106		(274		)	—		—		468,106		(274		)
Corporate debt securities	499,091		(932		)	—		—		499,091		(932		)
Municipal securities	11,042		(8		)	—		—		11,042		(8		)
Total	$	2,039,295	$	(1,587	)	$	—	$	—	$	2,039,295	$	(1,587	)

Schedule of investments by contractual maturity date

The following table presents available-for-sale investments by contractual maturity date as of December 31, 2015 (in thousands):

	Amortized Cost		Estimated Fair Market Value
Due in one year or less	$	1,653,937	$	1,652,844
Due after one year through two years	617,493		615,492
Due after two years through three years	305,911		304,809
Total	$	2,577,341	$	2,573,145