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LEASES - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Lessee, Lease, Description [Line Items]          
Operating lease cost $ 7.2 $ 20.7      
Lease commitments for leases not yet commenced   $ 16.8      
Operating leases expense     $ 27.0 $ 27.3 $ 22.9
Minimum          
Lessee, Lease, Description [Line Items]          
Lease term 1 year 1 year      
Lease not yet commenced term of contract 6 months 6 months      
Maximum          
Lessee, Lease, Description [Line Items]          
Lease term 22 years 22 years      
Lease not yet commenced term of contract 12 years 12 years