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Stock-Based Compensation - Stock Options (Details) - Common Shares - Stock Options - Long-Term Incentive Plans - USD ($) $ / shares in Units, shares in Thousands, $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2020
Stock compensation, number of options |  |  |
Stock options outstanding | 460 |  | 418
Stock compensation, other information |  |  |
Stock options outstanding - weighted average exercise price (USD per share) | $ 42.20 |  | $ 42.23
Aggregate intrinsic value, options exercised | $ 3 | $ 19 |

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