Marketable Securities (Tables)

6 Months Ended

Jun. 30, 2019

Investments Debt And Equity Securities [Abstract]

Summary of Marketable Securities Held

The following table summarizes the marketable securities held at June 30, 2019 and December 31, 2018 (in thousands):

Description	Amortized cost / Cost		Unrealized gains		Unrealized losses			Fair value
June 30, 2019
U.S. government agency securities and treasuries	$	1,060,606	$	3,084	$	(519	)	$	1,063,171
Certificates of deposit		4,520		—		—			4,520
Corporate bonds		113,820		633		(12	)		114,441
Commercial paper		62,692		—		—			62,692
Equity securities		20,017		—		(4,034	)		15,983
Total	$	1,261,655	$	3,717	$	(4,565	)	$	1,260,807
December 31, 2018
U.S. government agency securities and treasuries	$	1,459,649	$	963	$	(3,011	)	$	1,457,601
Certificates of deposit		9,080		—		—			9,080
Equity securities		20,017		2,150		—			22,167
Total	$	1,488,746	$	3,113	$	(3,011	)	$	1,488,848