Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Available-for-sale Securities

The following tables summarize the Company's investments in marketable securities:

	Amortized		Unrealized					Fair
As of December 31, 2017	Cost Basis		Gains		Losses			Value
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
Equity securities	56,191		6,594		—			62,785
	$	2,083,463	$	8,804	$	(8,926	)	$	2,083,341

As of December 31, 2016
Corporate bonds	$	1,076,964	$	630	$	(4,743	)	$	1,072,851
U.S. government and government agency obligations	132,923		58		(641		)	132,340
Municipal bonds	7,663		1		(20		)	7,644
Commercial paper	63,074		1		—			63,075
Certificates of deposit	42,612		—		—			42,612
Equity securities	57,251		5,551		(13,583		)	49,219
	$	1,380,487	$	6,241	$	(18,987	)	$	1,367,741

Marketable Securities, Based on Contractual Maturity Dates

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

	As of December 31,
	2017		2016
Maturities within one year	$	593,783	$	503,481
Maturities after one year through five years	1,426,773		815,041
	$	2,020,556	$	1,318,522

Fair Value and Unrealized Losses of Marketable Securities

The following table shows the fair value of the Company's marketable 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 December 31, 2017	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
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	)
As of December 31, 2016
Corporate bonds	$	759,222	$	(4,685	)	$	36,407	$	(58	)	$	795,629	$	(4,743	)
U.S. government and government agency obligations	81,170		(641		)	—		—			81,170		(641		)
Municipal bonds	7,141		(20		)	—		—			7,141		(20		)
Equity securities	36,417		(13,583		)	—		—			36,417		(13,583		)
	$	883,950	$	(18,929	)	$	36,407	$	(58	)	$	920,357	$	(18,987	)