FAIR VALUE MEASUREMENTS (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value of Assets Measured on a Recurring Basis

The following tables present information about our assets that are measured at fair value on a recurring basis at March 31, 2019 and December 31, 2018, and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value, in thousands:

Description	At March 31, 2019		Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents:
U.S. treasury securities	$	530,231	$	—	$	530,231	$	—
Money market funds		98,236		98,236		—		—
Marketable debt securities:
Certificates of deposit		3,000		—		3,000		—
Commercial paper		34,394		—		34,394		—
Corporate notes		113,253		—		113,253		—
U.S. government-sponsored enterprise securities		2,496		—		2,496		—
U.S. treasury securities		366,435		—		366,435		—
Restricted cash (money market funds)		1,478		1,478		—		—
Total	$	1,149,523	$	99,714	$	1,049,809	$	—

Description	At December 31, 2018		Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents:
U.S. treasury securities	$	221,281	$	—	$	221,281	$	—
Money market funds		102,445		102,445		—		—
Marketable debt securities:
Certificates of deposit		8,951		—		8,951		—
Commercial paper		57,197		—		57,197		—
Corporate notes		232,410		—		232,410		—
U.S. government-sponsored enterprise securities		39,018		—		39,018		—
U.S. treasury securities		325,227		—		325,227		—
Marketable equity securities		1,206		1,206		—		—
Restricted cash (money market funds)		1,477		1,477		—		—
Total	$	989,212	$	105,128	$	884,084	$	—