GOODWILL AND INTANGIBLE ASSETS - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in carrying amount of goodwill
Goodwill	$ 137,090	$ 101,315	$ 75,953
Acquired goodwill	35,775	25,362
Balance at the end of the period	137,090	$ 101,315
Goodwill impairment loss	$ 0