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Stock-Based Compensation - Stock Option Activity (Q3) (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          
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 period end (in shares) 291,169 802,609      
Weighted-Average Exercise Price per Share          
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 (in dollars per share) $ 49.75 $ 32.52      
Weighted-Average Remaining Contractual Life          
Outstanding, weighted average remaining contractual life (in years) 5 years 10 months 24 days 4 years 4 years 3 months 18 days 4 years 9 months 18 days 5 years 6 months
Exercisable weighted average remaining contractual life (in years) 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 $ 16,051 $ 36,848