| Summary of Activity Under Share-Based Compensation Plans | The following table summarizes the activity for stock appreciation rights, or SARs, under all share-based compensation plans for the nine months ended September 30, 2020:   
|   |   | Number of Awards |   |   | Weighted-Average Exercise Price Per Award |   |   | Weighted-Average Remaining Contractual Term |   | Aggregate Intrinsic Value(1) |   |  
|   |   | (in thousands) |   |   |   |   |   |   |   |   | (in millions) |   |  
| Outstanding as of December 31, 2019(2)(3) |   |   | 7,001 |   |   | $ | 27.85 |   |   | 5.4 years |   | $ | 138.7 |   |  
| Granted |   |   | — |   |   | $ | — |   |   |   |   |   |   |   |  
| Exercised(4) |   |   | (2,914 | ) |   | $ | 28.46 |   |   |   |   |   |   |   |  
| Forfeited |   |   | (7 | ) |   | $ | 29.03 |   |   |   |   |   |   |   |  
| Outstanding as of September 30, 2020(2)(3) |   |   | 4,080 |   |   | $ | 27.41 |   |   | 4.7 years |   | $ | 78.5 |   |  
| Exercisable as of September 30, 2020(5) |   |   | 4,074 |   |   | $ | 27.41 |   |   | 4.7 years |   | $ | 78.4 |   |  
| Vested and expected to vest as of September 30, 2020 |   |   | 4,080 |   |   | $ | 27.41 |   |   | 4.7 years |   | $ | 78.5 |   |    
| (1) | The intrinsic value is the amount by which the current market value of the underlying stock exceeds the exercise price of the SARs. |  
| (2) | Includes less than 0.1 million market condition SARs as of both September 30, 2020 and December 31, 2019. |  
| (3) | Includes 1.1 million and 2.9 million performance condition SARs as of September 30, 2020 and December 31, 2019, respectively, which represents the maximum amount that can vest. |  
| (4) | Includes 1.8 million performance condition SARs. |  
| (5) | Includes less than 0.1 million market condition and 1.1 million performance condition SARs. |  The following table summarizes the activities for stock units under all share-based compensation plans for the nine months ended September 30, 2020:   
|   |   | Number of Shares |   |   | Weighted-Average Grant Date Fair Value Per Share |   |  
|   |   | (in thousands) |   |   |   |   |   |  
| Outstanding and nonvested as of December 31, 2019(1) |   |   | 1,833 |   |   | $ | 49.49 |   |  
| Granted(2) |   |   | 1,975 |   |   | $ | 39.63 |   |  
| Vested |   |   | (321 | ) |   | $ | 49.32 |   |  
| Forfeited |   |   | (115 | ) |   | $ | 43.94 |   |  
| Outstanding and nonvested as of September 30, 2020(1) |   |   | 3,372 |   |   | $ | 43.92 |   |  
| Expected to vest as of September 30, 2020(3) |   |   | 3,094 |   |   | $ | 43.61 |   |    
| (1) | Includes 980,338 and 475,430 performance-based stock unit awards as of September 30, 2020 and December 31, 2019, respectively, which represents the maximum amount that can vest. |  
| (2) | Includes 504,908 performance-based stock unit awards. |  
| (3) | Includes 773,968 performance-based stock unit awards. |  |