CASH, CASH EQUIVALENTS, AND AVAILABLE-FOR-SALE SECURITIES (Tables)

9 Months Ended

Sep. 30, 2017

CASH, CASH EQUIVALENTS, AND AVAILABLE-FOR-SALE SECURITIES

Schedule of fair value and amortized cost of cash, cash equivalents, and available-for-sale securities by major security type

	As of September 30, 2017
			Gross		Gross
	Amortized		Unrealized		Unrealized		Estimated
Cash and cash equivalents and available-for-sale securities	Cost		Gains		Losses		Fair Value
Cash and money market funds	$	73,151	$	—	$	—	$	73,151
U.S. Treasury securities		17,621		—		(94)		17,527
Corporate debt securities		145,509		83		(247)		145,345
Total		236,281		83		(341)		236,023
Less amounts classified as cash and cash equivalents		(73,151)		—		—		(73,151)
Total available-for-sale securities	$	163,130	$	83	$	(341)	$	162,872

	As of December 31, 2016
			Gross		Gross
	Amortized		Unrealized		Unrealized		Estimated
Cash and cash equivalents and available-for-sale securities	Cost		Gains		Losses		Fair Value
Cash and money market funds	$	84,783	$	—	$	—	$	84,783
U.S. Treasury securities		24,780		7		(110)		24,677
Corporate debt securities		160,571		52		(564)		160,059
Total		270,134		59		(674)		269,519
Less amounts classified as cash and cash equivalents		(84,783)		—		—		(84,783)
Total available-for-sale securities	$	185,351	$	59	$	(674)	$	184,736

Schedule of Fair Value of Separate Accounts by Major Category of Investment

The following table represents the fair value hierarchy for our cash equivalents and available-for-sale securities by major security type (in thousands):

	As of September 30, 2017
	Level 1		Level 2		Level 3		Total
Cash and money market funds	$	73,151	$	—	$	—	$	73,151
U.S. Treasury securities		17,527		—		—		17,527
Corporate debt securities		—		145,345		—		145,345
Total	$	90,678	$	145,345	$	—	$	236,023

	As of December 31, 2016
	Level 1		Level 2		Level 3		Total
Cash and money market funds	$	84,783	$	—	$	—	$	84,783
U.S. Treasury securities		24,677		—		—		24,677
Corporate debt securities		—		160,059		—		160,059
Total	$	109,460	$	160,059	$	—	$	269,519