RELATED PERSON TRANSACTIONS - Atlantic Automotive Corporation and Leased Property by Real Estate Venture (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) real_estate_venture	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Advertising time | Atlantic Automotive
Related person transactions
Amount received	$ 0.6	$ 0.6	$ 0.4
Leased assets or facilities | Atlantic Automotive
Related person transactions
Amount received	$ 0.4	1.1	1.2
Number of real estate ventures | real_estate_venture	1
Real estate ventures in Baltimore, MD | Leased assets or facilities | Chief Executive Officer
Related person transactions
Amount received	$ 0.5	0.7	0.6
Number of real estate ventures | real_estate_venture	1
Amount paid	$ 0.1	0.1	0.1
Real Estate Ventures In Towson M D | Leased assets or facilities | Chief Executive Officer
Related person transactions
Amount received	$ 0.1	$ 0.4	$ 0.3