SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Provisions for depreciation
Depreciation	$ 27,381	$ 24,725	$ 19,354
Buildings | Minimum
Property, Plant and Equipment [Line Items]
Useful lives of the assets	10 years
Buildings | Maximum
Property, Plant and Equipment [Line Items]
Useful lives of the assets	40 years
Furniture, fixtures, and operating equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful lives of the assets	2 years
Furniture, fixtures, and operating equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful lives of the assets	10 years