FAIR VALUE MEASUREMENTS (Tables)

12 Months Ended

Dec. 31, 2017

FAIR VALUE MEASUREMENTS

Summary of the levels of the fair value hierarchy into which the Company's financial assets fall

The following table summarizes the levels of the fair value hierarchy into which the Company’s financial assets fall as of December 31, 2017:

	Total		Level 1		Level 2		Level 3
Financial assets:
Included in cash and cash equivalents
Government debt securities (1)	$	41,086	$	—	$	41,086	$	—
U.S. treasury instruments		39,978		—		39,978		—
Government securities money market		46,095		—		46,095		—
Investments
Government debt securities (1)		39,910		—		39,910		—
U.S. treasury instruments		55,935		7,943		47,992		—
Common stock		263		263		—		—
Total financial assets	$	223,267	$	8,206	$	215,061	$	—

(1) Consists of municipal bonds, agency bonds and agency discount notes.

The following table summarizes the levels of the fair value hierarchy into which the Company’s financial assets fall as of December 31, 2016:

	Total		Level 1		Level 2		Level 3
Financial assets:
Included in cash and cash equivalents
U.S. treasury bills	$	44,999	$	—	$	44,999	$	—
Government securities money market		201,934		—		201,934		—
Investments
U.S. treasury bills		32,995		—		32,995		—
Common stock		388		388		—		—
Total financial assets	$	280,316	$	388	$	279,928	$	—

Schedule of changes to the Company's investment classified as Level 3

	Convertible notes
January 1, 2016	$	635
Unrealized gains (losses) included in accumulated other comprehensive income		43
Transfer out of level 3 due to conversion of convertible notes to common stock		(678)
December 31, 2016	$	—
Unrealized gains (losses) related to investment still held as of December 31, 2016	$	—