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Income Taxes - Additional Information (Q3) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Jun. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Tax Disclosure [Abstract]                  
Initial analysis of tax law changes recorded net tax expense       $ 4,300   $ 4,300      
Provisional expense for Transition Tax       55,000   55,000      
Revaluation of deferred tax assets and liabilities       50,700   (50,700)      
Provisional benefit to net deferred taxes   $ 3,200       24,600      
Provisional transition tax on unrepatriated foreign earnings $ 3,300     $ 13,400   $ (51,800)      
Effective income tax rate (as a percent) 16.20%   15.70% 17.20% 23.40% 25.50% 24.10% 31.90%  
Reduction in liability for certain tax provisions $ 2,400         $ 0 $ 2,140 $ 694  
Favorable tax impact 1,100                
Liability for uncertain tax positions 4,500     $ 4,500   6,910 8,423 $ 10,557 $ 12,596
Unrecognized tax benefits if recognized that would affect effective tax rate 3,300     3,300   4,400 4,200    
Reduction in accrued interest expense 300                
Accrued interest related to uncertain tax positions 600     600   700 800    
Accrued penalties related to uncertain tax positions 100     100   100 $ 300    
Unrecognized tax benefits subject to change within the next 12 months $ 4,500     $ 4,500   $ 5,200