Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of assets measured at fair value on a recurring basis

The Company's assets that are measured at fair value on a recurring basis consist of the following:

			Fair Value Measurements at Reporting Date Using
As of December 31, 2017	Fair Value		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)
Available-for-sale marketable securities:
Corporate bonds	$	1,712,480	—		$	1,712,480
U.S. government and government agency obligations	185,492		—		185,492
Municipal bonds	4,587		—		4,587
Commercial paper	106,973		—		106,973
Certificates of deposit	11,024		—		11,024
Equity securities	62,785		$	62,785	—
	$	2,083,341	$	62,785	$	2,020,556
As of December 31, 2016
Available-for-sale marketable securities:
Corporate bonds	$	1,072,851	—		$	1,072,851
U.S. government and government agency obligations	132,340		—		132,340
Municipal bonds	7,644		—		7,644
Commercial paper	63,075		—		63,075
Certificates of deposit	42,612		—		42,612
Equity securities	49,219		$	49,219	—
	$	1,367,741	$	49,219	$	1,318,522