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Leases - Narrative (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Lessee, Lease, Description [Line Items]      
Renewal term 2 years    
Termination period 1 year    
Rent expense   $ 4.2 $ 2.9
Minimum      
Lessee, Lease, Description [Line Items]      
Remaining terms 1 year    
Maximum      
Lessee, Lease, Description [Line Items]      
Remaining terms 4 years