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Leases (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Lessee, Lease, Description [Line Items]      
Operating leases, rent expense   $ 118 $ 126
Minimum      
Lessee, Lease, Description [Line Items]      
Lease term 1 year    
Maximum      
Lessee, Lease, Description [Line Items]      
Lease term 11 years