Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 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 March 31, 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	$	2,110,609	—		$	2,110,609
U.S. government and government agency obligations	154,700		—		154,700
Municipal bonds	2,580		—		2,580
Commercial paper	70,385		—		70,385
Certificates of deposit	20,097		—		20,097
Equity securities	69,075		$	69,075	—
	$	2,427,446	$	69,075	$	2,358,371
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