The following tables set forth our financial assets as of June 30, 2019 and December 31, 2018 that are measured at fair value on a recurring basis during the period:

	June 30, 2019
	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Total Estimated Fair Value
	(in thousands)
Cash	$	275,644	$	—	$	—		$	275,644
Level 1 securities:
U.S. government securities		630,566		541		(12	)		631,095
U.S. government agency securities		154,109		39		(19	)		154,129
Level 2 securities:
Corporate debt securities		30,713		56		—			30,769
Commercial paper		55,706		—		—			55,706
Certificates of deposit		37,451		—		—			37,451
Total	$	1,184,189	$	636	$	(31	)	$	1,184,794

	December 31, 2018
	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Total Estimated Fair Value
	(in thousands)
Cash	$	279,950	$	—	$	—		$	279,950
Level 1 securities:
U.S. government securities		735,988		12		(175	)		735,825
U.S. government agency securities		181,032		4		(36	)		181,000
Level 2 securities:
Corporate debt securities		35,819		1		(18	)		35,802
Commercial paper		33,193		—		—			33,193
Certificates of deposit		13,293		—		—			13,293
Total	$	1,279,275	$	17	$	(229	)	$	1,279,063