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EMPLOYEE BENEFIT PLANS - Weighted Average Assumptions Used to Develop Actuarial Present Value of Projected Benefit Obligation (FY) (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
United States | Defined Benefit Pension Plans          
Defined Benefit Plan Disclosure [Line Items]          
Discount rate (as a percent) 3.56% 3.95% 3.56% 3.95% 4.21%
Expected return on plan assets (as a percent) 5.15% 4.95% 4.95% 5.70% 5.70%
Rate of compensation increase (as a percent) 3.00% 3.00% 3.00% 3.00% 3.00%
United States | Other Postretirement Benefits Plans          
Defined Benefit Plan Disclosure [Line Items]          
Discount rate (as a percent) 3.43% 3.76% 3.43% 3.76% 3.95%
Foreign Plan | Defined Benefit Pension Plans          
Defined Benefit Plan Disclosure [Line Items]          
Discount rate (as a percent) 2.40% 2.51% 2.40% 2.51% 3.56%
Expected return on plan assets (as a percent) 5.10% 4.93% 5.02% 6.07% 5.81%
Rate of compensation increase (as a percent) 2.60% 2.54% 2.54% 2.54% 3.10%
Foreign Plan | Other Postretirement Benefits Plans          
Defined Benefit Plan Disclosure [Line Items]          
Discount rate (as a percent) 3.21% 3.46% 3.21% 3.46% 3.80%