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Stock-Based Compensation (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Share-based Compensation [Line Items]      
Service period     3 years
Stock options      
Share-based Compensation [Line Items]      
Vesting period     3 years
Expiration period     7 years
Equity-classified awards, unrecognized compensation cost $ 11   $ 11
Weighted average period over which unrecognized cost is recognized, years     1 year 3 months 18 days
Restricted stock      
Share-based Compensation [Line Items]      
Vesting period     4 years
RSUs      
Share-based Compensation [Line Items]      
Vesting period 4 years 4 years  
Weighted average period over which unrecognized cost is recognized, years     3 years 2 months 12 days
Liability classified awards, unrecognized compensation costs $ 38   $ 38
Performance units      
Share-based Compensation [Line Items]      
Vesting period 3 years 3 years 3 years
Weighted average period over which unrecognized cost is recognized, years     2 years 4 months 24 days
Liability classified awards, unrecognized compensation costs $ 16   $ 16