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Shareholders' Equity and Long-Term Incentive Compensation - Stock Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions |  |  |  | 12 Months Ended
--- | --- | --- | --- | ---
Feb. 20, 2020 | Feb. 21, 2019 | Feb. 22, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Stock Options |  |  |  |  |  |
Historical period upon which annual expected stock price volatility is based (years) |  |  |  | 3 years |  |
Maximum |  |  |  |  |  |
Stock Options |  |  |  |  |  |
Daily treasury yield curve period upon which average risk-free interest rate is based (years) |  |  |  | 3 years |  |
PSU's, RSU's, and RS's |  |  |  |  |  |
Unvested Restricted Stock Shares and Restricted Stock Units |  |  |  |  |  |
Outstanding at the beginning of the period (in shares) |  |  |  | 606 | 652 | 1,230
Granted (in shares) |  |  |  | 277 | 430 | 211
Vested (in shares) |  |  |  | (233) | (446) | (753)
Forfeited (in shares) |  |  |  | (6) | (30) | (36)
Outstanding at the end of the period (in shares) |  |  |  | 644 | 606 | 652
Weighted-Average Grant-Date Fair Value |  |  |  |  |  |
Outstanding at the beginning of the period (in dollars per share) |  |  |  | $ 36.17 | $ 24,650,000 | $ 17.41
Granted (in dollars per share) |  |  |  | 46.61 | 35.49 | 33.69
Vested (in dollars per share) |  |  |  | 31.49 | 18.75 | 15.39
Forfeited (in dollars per share) |  |  |  | 41.37 | 35.10 | 22.35
Outstanding at the end of the period (in dollars per share) |  |  |  | $ 42.32 | $ 36.17 | $ 24,650,000
Unrecognized compensation cost |  |  |  |  |  |
Unrecognized compensation cost related to PSUs, RSUs, and RSs |  |  |  | $ 10.7 |  |
PSU's, RSU's, and RS's | Maximum |  |  |  |  |  |
Stock Options |  |  |  |  |  |
Vesting period (years) |  |  |  | 3 years |  |
PSU's, RSU's, and RS's | Weighted Average |  |  |  |  |  |
Unrecognized compensation cost |  |  |  |  |  |
Weighted-average period over which unrecognized compensation costs will be recognized |  |  |  | 1 year 9 months 18 days |  |
Stock options |  |  |  |  |  |
Stock Options |  |  |  |  |  |
Granted (in shares) | 125 | 186 | 184 | 139 | 189 | 184
Exercise price per share (in dollars per share) | $ 50.09 | $ 36.51 | $ 32.69 | $ 49.57 | $ 36.50 | $ 32.69
Fair value of stock option grants (in dollars per share) | $ 17.40 | $ 13.31 | $ 11.66 |  |  |
Historical period upon which annual expected stock price volatility is based (years) | 6 years |  |  |  |  |
Vesting period (years) | 3 years |  |  |  |  |
Stock option activity, Shares |  |  |  |  |  |
Options outstanding and exercisable at the beginning of the period (in shares) |  |  |  | 1,692 | 1,718 | 1,608
Exercised (in shares) |  |  |  | (412) | (202) | (64)
Forfeited (in shares) |  |  |  | 0 | (13) | (10)
Granted (in shares) | 125 | 186 | 184 | 139 | 189 | 184
Options outstanding and exercisable at the end of the period (in shares) |  |  |  | 1,419 | 1,692 | 1,718
Stock option activity, Weighted Average Exercise Price |  |  |  |  |  |
Options outstanding and exercisable, beginning balance (in dollars per share) |  |  |  | $ 19.05 | $ 16.58 | $ 14.67
Exercised, Weighted Average Grant Date Fair Value (in dollars per share) |  |  |  | 14.97 | 13.46 | 13.89
Forfeited, Weighted Average Exercise Price (in dollars per share) |  |  |  | 0 | 33.15 | 23.17
Granted, Weighted Average Exercise Price (in dollars per share) | $ 50.09 | $ 36.51 | $ 32.69 | 49.57 | 36.50 | 32.69
Options outstanding and exercisable, ending balance (in dollars per share) |  |  |  | $ 23.21 | $ 19.05 | $ 16.58
Number of stock options exercisable (in shares) |  |  |  | 1,100 |  |
Compensation costs not yet recognized |  |  |  | $ 1.5 |  |
Stock options | Maximum |  |  |  |  |  |
Stock Options |  |  |  |  |  |
Maximum contractual term (years) | 10 years |  |  |  |  |
Daily treasury yield curve period upon which average risk-free interest rate is based (years) | 7 years |  |  |  |  |
Stock options | Weighted Average |  |  |  |  |  |
Unrecognized compensation cost |  |  |  |  |  |
Weighted-average period over which unrecognized compensation costs will be recognized |  |  |  | 2 years |  |

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