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COMMITMENTS AND CONTINGENCIES - Narrative (Details)
12 Months Ended
Sep. 08, 2017
USD ($)
Nov. 02, 2016
patent
Apr. 21, 2016
patent
Dec. 31, 2017
USD ($)
claim
Pending Litigation        
Loss Contingencies [Line Items]        
Pending litigation cases       0
Siemens Industry, Inc. | Pending Litigation        
Loss Contingencies [Line Items]        
Patents allegedly infringed | patent   6 7  
Denver Transit Delivery of Train Management System | Threatened Litigation | Subsidiaries        
Loss Contingencies [Line Items]        
Alleged damages | $       $ 0
Denver Transit Installation of Constant Wireless Crossings | Threatened Litigation | Subsidiaries        
Loss Contingencies [Line Items]        
Alleged damages | $ $ 36,800,000      
Denver Transit | Threatened Litigation | Subsidiaries        
Loss Contingencies [Line Items]        
Claims filed       0
Asbestos Related Cases | Settled Litigation        
Loss Contingencies [Line Items]        
Arbitration decisions       2