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Stock-Based Compensation (Details) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($) item $ / shares shares | Dec. 31, 2019 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2014
Stock-based compensation |  |  |  |
Total unrecognized compensation cost related to the unvested awards | $ | $ 13.2 |  |  |
Total weighted average remaining term of unrecognized compensation costs | 1 year 5 months 23 days |  |  |
Compensation cost recognized | $ | $ 12.7 | $ 17.8 | $ 13.8 |
Class A |  |  |  |
Stock-based compensation |  |  |  |
Shares available for future grants of new equity awards | shares | 1,083,096 |  |  |
Performance stock units |  |  |  |
Stock-based compensation |  |  |  |
Granted (in shares) | shares | 94,000 | 88,000 | 96,000 |
Total unrecognized compensation cost related to the unvested awards | $ | $ 5.5 |  |  |
Total weighted average remaining term of unrecognized compensation costs | 1 year 5 months 26 days |  |  |
Compensation cost recognized | $ | $ 4.3 | $ 8.5 | $ 5.2 |
Fair value assumptions |  |  |  |
Weighted average grant-date fair value (in dollars per share) | $ 70.65 | $ 77.58 | $ 81.51 |
Performance stock units | Class A |  |  |  |
Weighted Average Intrinsic Value |  |  |  |
Company's closing Common Stock price (in dollars per share) | $ 121.70 |  |  |
Previous Stock Incentive Plan 2004 |  |  |  |
Stock-based compensation |  |  |  |
Expiration period (years) | 10 years |  |  |
Minimum exercise price as percentage of fair market value of common stock on grant date |  |  |  | 100.00%
Second Amended and Restated 2004 Stock Incentive Plan |  |  |  |
Stock-based compensation |  |  |  |
Number of stock incentive plans | item | 1 |  |  |
Second Amended and Restated 2004 Stock Incentive Plan | Stock options |  |  |  |
Stock-based compensation |  |  |  |
Total unrecognized compensation cost related to the unvested awards | $ | $ 0.0 |  |  |
Summary of stock option activity and related information |  |  |  |
Outstanding at beginning of year (in shares) | shares | 10,000 | 49,000 | 95,000 |
Cancelled/Forfeitures (in shares) | shares |  | (1,000) |  |
Exercised (in shares) | shares | (5,000) | (38,000) | (46,000) |
Outstanding at end of year (in shares) | shares | 5,000 | 10,000 | 49,000 |
Exercisable at end of year (in shares) | shares | 5,000 | 10,000 | 49,000 |
Weighted Average Exercise Price |  |  |  |
Outstanding at beginning of year (in dollars per share) | $ 53.65 | $ 55.25 | $ 54.91 |
Cancelled/Forfeitures (in dollars per share) |  | 57.47 |  |
Exercised (in dollars per share) | 55.03 | 55.63 | 54.55 |
Outstanding at end of year (in dollars per share) | 52.40 | 53.65 | 55.25 |
Exercisable at end of year (in dollars per share) | 52.40 | $ 53.65 | $ 55.25 |
Weighted Average Intrinsic Value |  |  |  |
Outstanding at end of year (in dollars per share) | 69.30 |  |  |
Exercisable at end of year (in dollars per share) | $ 69.30 |  |  |
Aggregate intrinsic values of exercisable options (in dollars) | $ | $ 0.4 |  |  |
Total intrinsic value of options exercised | $ | $ 0.3 | $ 1.3 | $ 1.2 |
Second Amended and Restated 2004 Stock Incentive Plan | Stock options | Class A |  |  |  |
Weighted Average Intrinsic Value |  |  |  |
Company's closing Common Stock price (in dollars per share) | $ 121.70 |  |  |
Second Amended and Restated 2004 Stock Incentive Plan | Restricted Stock and Deferred Shares |  |  |  |
Stock-based compensation |  |  |  |
Vesting period | 3 years |  |  |
Percentage of stock options becoming exercisable | 33.00% |  |  |
Second Amended and Restated 2004 Stock Incentive Plan | Performance Shares and Deferred Shares |  |  |  |
Stock-based compensation |  |  |  |
Minimum service period | 10 years |  |  |
Minimum age | 55 years |  |  |
Second Amended and Restated 2004 Stock Incentive Plan | Performance stock units |  |  |  |
Stock-based compensation |  |  |  |
Vesting period | 3 years |  |  |
Period of time used to calculate the compound annual growth rate | 3 years |  |  |
Second Amended and Restated 2004 Stock Incentive Plan | Performance stock units | Minimum |  |  |  |
Stock-based compensation |  |  |  |
Percent of target shares a recipient may earn | 0 |  |  |
Second Amended and Restated 2004 Stock Incentive Plan | Performance stock units | Maximum |  |  |  |
Stock-based compensation |  |  |  |
Percent of target shares a recipient may earn | 2 |  |  |
Management Stock Purchase Plan |  |  |  |
Stock-based compensation |  |  |  |
Percentage of annual incentive bonus that may be used to purchase RSU's | 50.00% |  |  |
Purchase price as percentage of fair market value of common stock on grant date | 80.00% |  |  |
Management Stock Purchase Plan | Class A |  |  |  |
Stock-based compensation |  |  |  |
Shares authorized | shares | 2,000,000 |  |  |
Shares available for future grants of new equity awards | shares | 731,163 |  |  |
Management Stock Purchase Plan | Restricted stock |  |  |  |
Stock-based compensation |  |  |  |
Compensation cost recognized | $ | $ 0.7 | $ 0.8 |  |
Management Stock Purchase Plan | Restricted stock | Class A |  |  |  |
Weighted Average Intrinsic Value |  |  |  |
Company's closing Common Stock price (in dollars per share) | $ 121.70 |  |  |
Management Stock Purchase Plan | Restricted stock units (RSUs) |  |  |  |
Stock-based compensation |  |  |  |
Minimum deferral period | 3 years |  |  |
Granted (in shares) | shares | 28,000 | 37,000 | 36,000 |
Total unrecognized compensation cost related to the unvested awards | $ | $ 0.8 |  |  |
Total weighted average remaining term of unrecognized compensation costs | 1 year 4 months 20 days |  |  |
Compensation cost recognized | $ |  |  | $ 1.0 |
Fair value assumptions |  |  |  |
Expected life (years) | 3 years | 3 years |  |
Expected stock price volatility (as a percent) | 24.60% | 23.30% |  |
Expected dividend yield (as a percent) | 1.10% | 1.10% |  |
Risk-free interest rate (as a percent) | 0.60% | 2.50% |  |
Weighted average grant-date fair value (in dollars per share) | $ 22.36 | $ 22.16 | $ 21.80 |
Management Stock Purchase Plan | Restricted stock units (RSUs) | Minimum |  |  |  |
Stock-based compensation |  |  |  |
Vesting period | 3 years |  |  |

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