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STOCK-BASED COMPENSATION PLANS - Stock Option Activity (FY) (Details) - USD ($)
$ / shares in Units, $ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Options          
Outstanding, beginning of period (in shares) 983,512 1,098,823 1,097,323 1,147,558  
Granted (in shares) 82,580 65,522 94,115 84,675  
Exercised (in shares) (574,303) (166,838) (83,790) (124,156)  
Canceled (in shares) (16,471) (13,995) (8,825) (10,754)  
Outstanding, end of period (in shares) 475,318 983,512 1,098,823 1,097,323 1,147,558
Exercisable at December 31, 2017 291,169 802,609      
Weighted Average Exercise Price          
Outstanding, beginning of period (in dollars per share) $ 40.62 $ 35.39 $ 32.70 $ 28.33  
Granted (in dollars per share) 77.54 86.91 61.39 87.35  
Exercised (in dollars per share) 23.88 21.37 25.58 26.70  
Canceled (in dollars per share) 64.46 76.89 71.47 65.22  
Outstanding, end of period (in dollars per share) 56.50 40.62 $ 35.39 $ 32.70 $ 28.33
Exercisable at December 31, 2017 (in dollars per share) $ 49.75 $ 32.52      
Weighted Average Remaining Contractual Life          
Weighted- Average Remaining Contractual Life, outstanding 5 years 10 months 24 days 4 years 4 years 3 months 18 days 4 years 9 months 18 days 5 years 6 months
Weighted Average Remaining Contractual Life, Exercisable shares at December 31, 2017 5 years 3 years 3 months 18 days      
Aggregate Intrinsic Value          
Outstanding, beginning of period $ 40,137 $ 52,332 $ 42,154 $ 67,205  
Granted 2,258 0 2,035 1,375  
Exercised (46,520) (10,020) (4,813) (5,516)  
Canceled (666) (64) (102) (64)  
Outstanding, end of period 22,997 40,137 $ 52,332 $ 42,154 $ 67,205
Exercisable at December 31, 2017 $ 16,051 $ 36,848