INVESTMENTS - Unrealized losses and fair values of available-for-sale securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Securities in an unrealized loss position less than 12 months
Unrealized Losses Less Than 12 months	$ (37)	$ (21)
Estimated Fair Value Less Than 12 Months	10,176	11,017
Total Unrealized Losses	(37)	(21)
Total Estimated Fair Value	10,176	11,017
Other-than-temporary impairment loss
Other-than-temporary impairment loss recognized	0
Commercial paper and corporate bonds
Securities in an unrealized loss position less than 12 months
Unrealized Losses Less Than 12 months	(37)	(20)
Estimated Fair Value Less Than 12 Months	10,176	7,520
Total Unrealized Losses	(37)	(20)
Total Estimated Fair Value	$ 10,176	7,520
U.S. government agency debt securities
Securities in an unrealized loss position less than 12 months
Unrealized Losses Less Than 12 months		(1)
Estimated Fair Value Less Than 12 Months		3,497
Total Unrealized Losses		(1)
Total Estimated Fair Value		$ 3,497