SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Advertising Expense (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Advertising expenses	$ 213.8	$ 201.5	$ 132.7
Advertising expense, barter costs	$ 155.2	$ 146.1	$ 68.9