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INCOME TAXES - Narrative (FY) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Tax Disclosure [Abstract]            
Net tax expense recorded   $ 4,300 $ 4,300      
Provisional expense for U.S. tax reform bill   55,000 55,000      
Net benefit for revaluation of deferred tax assets and liabilities   (50,700) 50,700      
Provisional benefit for net deferred taxes $ 3,200   24,600      
Transition tax expense     $ 51,800      
France tax rate change (as a percent)     6.50% 6.50% (0.00%)  
U.S. tax rate change     7.90% (0.00%) (0.00%)  
U.S. tax reform provision     15.60% 0.00% 0.00%  
Transaction costs related to acquisitions     0.00% 1.50% 0.00%  
Valuation allowance for foreign carryforwards     $ 25,700      
Net operating loss carryforwards     65,700      
Unrecognized tax benefits   4,500 6,910 $ 8,423 $ 10,557 $ 12,596
Unrecognized tax benefits if recognized that would affect effective tax rate   3,300 4,400 4,200    
Accrued interest   600 700 800    
Accrued penalties   100 100 $ 300    
Unrecognized tax benefits subject to change within the next 12 months   $ 4,500 $ 5,200