SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Leases (Narrative) (Details)	12 Months Ended
	Dec. 31, 2016
Street furniture display faces
Operating Leased Assets [Line Items]
Contract term	15 years
Tower site leases
Operating Leased Assets [Line Items]
Contract term	30 years
Americas | Outdoor land leases | Minimum
Operating Leased Assets [Line Items]
Contract term	1 month
Americas | Outdoor land leases | Maximum
Operating Leased Assets [Line Items]
Contract term	12 months
International | Outdoor land leases | Maximum
Operating Leased Assets [Line Items]
Contract term	12 months