Fair Value Measurements (Tables)

9 Months Ended

Sep. 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 September 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	$	2,546,243	—		$	2,546,243
U.S. government and government agency obligations	110,548		—		110,548
Municipal bonds	2,063		—		2,063
Commercial paper	109,413		—		109,413
Certificates of deposit	65,940		—		65,940
Equity securities (unrestricted)	90,445		$	90,445	—
Equity securities (restricted)	53,226		—		53,226
	$	2,977,878	$	90,445	$	2,887,433
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 (unrestricted)	62,785		$	62,785	—
	$	2,083,341	$	62,785	$	2,020,556