INTANGIBLE ASSETS - Intangible Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Carrying amount:
Total carrying amount	$ 126,066	$ 126,066
Accumulated amortization:
Total accumulated amortization	(45,664)	(39,361)
Total intangible assets, net	80,402	86,705
Customer service agreements
Carrying amount:
Total carrying amount	125,960	125,960
Accumulated amortization:
Total accumulated amortization	(45,626)	(39,328)
Other
Carrying amount:
Total carrying amount	106	106
Accumulated amortization:
Total accumulated amortization	$ (38)	$ (33)