SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognitions (Narrative) (Details)	12 Months Ended
	Dec. 31, 2018
Americas and International Outdoor Advertising
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, performance obligation, description of timing	Revenues from these contracts, which typically cover periods of a few weeks to one year, are generally recognized ratably over the term of the contract as the advertisement is displayed
Other
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, performance obligation, description of timing	The Company’s media representation contracts typically have terms up to ten years in duration and are generally billed monthly upon satisfaction of the performance obligations