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Leases - Narrative (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Leases [Line Items]    
Cumulative effect of accounting change   $ 176
Retained earnings    
Leases [Line Items]    
Cumulative effect of accounting change   $ 175
ASC 842 | Retained earnings    
Leases [Line Items]    
Cumulative effect of accounting change $ 0