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CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounts Payable      
Purchase of property and equipment incurred $ 5.2 $ 6.6 $ 0.4
Accrued Expenses      
Purchase of property and equipment incurred $ 2.7