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Stock-Based Compensation - Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Stock based compensation expense      
Total stock-based compensation expense, before income taxes $ 41 $ 37 $ 30
Income tax benefit (10) (9) (11)
Total stock-based compensation expense, net of income taxes 31 28 19
Unrecognized compensation cost for all stock-based compensation awards $ 29    
Weighted average period for recognition of unrecognized compensation cost for all stock-based compensation awards 1 year 9 months 18 days    
Tax benefit from exercise of stock options and other awards $ 7    
Selling, general and administrative expense      
Stock based compensation expense      
Total stock-based compensation expense, before income taxes 36 33 27
System operations expense      
Stock based compensation expense      
Total stock-based compensation expense, before income taxes 5 4 3
Long-Term Incentive Plans | Stock Options      
Stock based compensation expense      
Total stock-based compensation expense, before income taxes 0 2 6
Long-Term Incentive Plans | Restricted Stock Units      
Stock based compensation expense      
Total stock-based compensation expense, before income taxes 22 21 19
Long-Term Incentive Plans | Performance Shares      
Stock based compensation expense      
Total stock-based compensation expense, before income taxes 18 13 4
Non-Employee Directors' Plan      
Stock based compensation expense      
Total stock-based compensation expense, before income taxes $ 1 $ 1 $ 1