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Summary of Significant Accounting Policies - Stock Split (Details)
$ in Millions
3 Months Ended 12 Months Ended
Feb. 21, 2018
Mar. 31, 2018
Dec. 31, 2018
USD ($)
Class of Stock [Line Items]      
Stock split ratio 2 2  
Common Stock      
Class of Stock [Line Items]      
Adjustments due stock split, APIC     $ 4
Additional Paid-In Capital      
Class of Stock [Line Items]      
Adjustments due stock split, APIC     $ (4)