SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recently Adopted Accounting Pronouncements (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets	$ 17,796	$ 22,000	$ 0
Operating lease liabilities	$ 25,048	28,600
Adjustment for ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued lease payments		7,200
Prepaid rent		$ 600