COMMITMENTS AND CONTINGENCIES - Lease Commitments (Details) $ in Thousands, ft² in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
COMMITMENTS AND CONTINGENCIES
Area of lease (in square feet) | ft²	2.5
Obligation for payments under leases
2018	$ 24,385
2019	21,692
2020	17,332
2021	9,823
2022	5,868
Thereafter	9,087
Total obligations under operating leases	88,187
Rent expense
Rent expense	$ 20,900	$ 19,800	$ 17,000