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Stock-Based Compensation - Expense (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Stock based compensation expense |  |  |
Total stock-based compensation expense, before income taxes | $ 32 | $ 41 | $ 37
Income tax benefit | (8) | (10) | (9)
Total stock-based compensation expense, net of income taxes | 24 | 31 | 28
Unrecognized compensation cost for all stock-based compensation awards | $ 33 |  |
Weighted average period for recognition of unrecognized compensation cost for all stock-based compensation awards | 2 years 1 month 6 days |  |
Tax benefit from exercise of stock options and other awards | $ 12 |  |
Selling, general and administrative expense |  |  |
Stock based compensation expense |  |  |
Total stock-based compensation expense, before income taxes | 28 | 36 | 33
System operations expense |  |  |
Stock based compensation expense |  |  |
Total stock-based compensation expense, before income taxes | 4 | 5 | 4
Long-Term Incentive Plans | Stock Options |  |  |
Stock based compensation expense |  |  |
Total stock-based compensation expense, before income taxes | 0 | 0 | 2
Long-Term Incentive Plans | Restricted Stock Units |  |  |
Stock based compensation expense |  |  |
Total stock-based compensation expense, before income taxes | 21 | 22 | 21
Long-Term Incentive Plans | Performance Shares |  |  |
Stock based compensation expense |  |  |
Total stock-based compensation expense, before income taxes | 10 | 18 | 13
Non-Employee Directors' Plan |  |  |
Stock based compensation expense |  |  |
Total stock-based compensation expense, before income taxes | $ 1 | $ 1 | $ 1

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