Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2018

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 June 30, 2018	Fair Value		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)
Available-for-sale debt securities:
Corporate bonds	$	2,480,571	—		$	2,480,571
U.S. government and government agency obligations	124,836		—		124,836
Municipal bonds	2,564		—		2,564
Commercial paper	76,013		—		76,013
Certificates of deposit	40,773		—		40,773
Equity securities	85,588		$	85,588	—
	$	2,810,345	$	85,588	$	2,724,757
As of December 31, 2017
Available-for-sale debt 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