SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Total	$ 322,985	$ 386,371
U.S. treasury securities
Total	229,423	265,393
U.S. government agency securities
Total	93,562	120,978
Level 1
Total	322,985	386,371
Level 1 | U.S. treasury securities
Total	229,423	265,393
Level 1 | U.S. government agency securities
Total	$ 93,562	$ 120,978