Marketable Securities (Tables)

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Available-for-sale Securities

The following tables summarize the Company's investments in available-for-sale debt securities:

	Amortized		Unrealized					Fair
As of March 31, 2018	Cost Basis		Gains		Losses			Value
Available-for-sale debt securities:
Corporate bonds	$	2,126,820	$	1,948	$	(18,159	)	$	2,110,609
U.S. government and government agency obligations	156,281		30		(1,611		)	154,700
Municipal bonds	2,591		—		(11		)	2,580
Commercial paper	70,385		—		—			70,385
Certificates of deposit	20,097		—		—			20,097
	$	2,376,174	$	1,978	$	(19,781	)	$	2,358,371
As of December 31, 2017
Available-for-sale debt securities:
Corporate bonds	$	1,717,976	$	2,176	$	(7,672	)	$	1,712,480
U.S. government and government agency obligations	186,699		34		(1,241		)	185,492
Municipal bonds	4,600		—		(13		)	4,587
Commercial paper	106,973		—		—			106,973
Certificates of deposit	11,024		—		—			11,024
	$	2,027,272	$	2,210	$	(8,926	)	$	2,020,556

Marketable Securities, Based on Contractual Maturity Dates

The fair values of debt security investments by contractual maturity consist of the following:

	March 31, 2018		December 31, 2017
Maturities within one year	$	605,461	$	593,783
Maturities after one year through five years	1,752,910		1,426,773
	$	2,358,371	$	2,020,556

Fair Value and Unrealized Losses of Marketable Securities

The following table shows the fair value of the Company's debt securities that have unrealized losses and that are deemed to be only temporarily impaired, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position.

	Less than 12 Months					12 Months or Greater					Total
As of March 31, 2018	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
Corporate bonds	$	1,343,778	$	(13,950	)	$	234,258	$	(4,209	)	$	1,578,036	$	(18,159	)
U.S. government and government agency obligations	70,872		(623		)	76,793		(988		)	147,665		(1,611		)
Municipal bonds	2,579		(11		)	—		—			2,579		(11		)
	$	1,417,229	$	(14,584	)	$	311,051	$	(5,197	)	$	1,728,280	$	(19,781	)
As of December 31, 2017
Corporate bonds	$	930,970	$	(4,924	)	$	256,750	$	(2,748	)	$	1,187,720	$	(7,672	)
U.S. government and government agency obligations	110,532		(409		)	67,921		(832		)	178,453		(1,241		)
Municipal bonds	2,582		(10		)	2,005		(3		)	4,587		(13		)
	$	1,044,084	$	(5,343	)	$	326,676	$	(3,583	)	$	1,370,760	$	(8,926	)