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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Advertising, Equity Offering Costs and Capitalized Interest (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Advertising and Marketing Costs      
Advertising and marketing expenses $ 9.7 $ 9.4 $ 8.6
Equity Offering Costs      
Equity offering costs 0.6 1.6 2.5
Capitalized Interest      
Interest costs, capitalized $ 5.6 $ 4.6 $ 2.6