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INCOME TAXES - Liability for Income Taxes Associated with Uncertain Tax Positions (FY) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Gross liability for unrecognized tax benefits at beginning of year   $ 8,423 $ 10,557 $ 12,596
Gross increases - unrecognized tax benefits in prior periods   2,466 6 0
Gross increases - current period unrecognized tax benefits   0 0 1,682
Gross decreases - unrecognized tax benefits in prior periods   0 0 0
Gross decreases - audit settlement during year   (3,979) 0 (3,027)
Gross decreases - expiration of audit statute of limitations $ (2,400) 0 (2,140) (694)
Gross liability for unrecognized tax benefits at end of year $ 4,500 $ 6,910 $ 8,423 $ 10,557