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Stock-Based Compensation Plans (Information Regarding Restricted Shares) (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Restricted Stock |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
General vesting period (years) | 3 years |  |
Unearned compensation costs | $ 18.5 | $ 18.0 |
Recognition of unearned compensation, (years) | 1 year |  |
Fair value of Restricted Shares vested | $ 24.2 | 23.5 | $ 25.2
Stock-based compensation expense | 27.8 | 25.5 | 29.5
Related income tax benefit | (6.3) | (5.8) | (6.7)
Net stock-based compensation expense | $ 21.5 | $ 19.7 | $ 22.8
Shares |  |  |
Outstanding — beginning of year (in shares) | 1,697,779 |  |
Granted (in shares) | 936,683 |  |
Vested (in shares) | (645,946) |  |
Canceled (in shares) | (247,030) |  |
Outstanding — ending of year (in shares) | 1,741,486 | 1,697,779 |
Weighted Average Grant-Date Fair Value |  |  |
Outstanding — beginning of year (in dollars per share) | $ 37.17 |  |
Granted (in dollars per share) | 36.72 |  |
Vested (in dollars per share) | 37.48 |  |
Canceled (in dollars per share) | 42.42 |  |
Outstanding — ending of year (in dollars per share) | $ 36.06 | $ 37.17 |
Restricted Stock | Minimum |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
General expiration period, from date of grant or time of termination (years) | 1 year |  |
General vesting period (years) | 1 year |  |
Restricted Stock | Maximum |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
General expiration period, from date of grant or time of termination (years) | 3 years |  |
General vesting period (years) | 3 years |  |
Performance Shares |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
General vesting period (years) | 36 months |  |
Shares |  |  |
Outstanding — ending of year (in shares) | 473,000 |  |
Weighted Average Grant-Date Fair Value |  |  |
Total shareholder return performance measure, as a percentage | 15.00% |  |
Estimated vesting of shares based on performance shares | 370,000 |  |
Performance Shares | Minimum |  |  |
Weighted Average Grant-Date Fair Value |  |  |
Estimated vesting of shares based on performance shares | 0 |  |
Performance Shares | Maximum |  |  |
Weighted Average Grant-Date Fair Value |  |  |
Estimated vesting of shares based on performance shares | 1,087,000 |  |
Performance Shares | Maximum | Issued in 2021 |  |  |
Weighted Average Grant-Date Fair Value |  |  |
Vesting percentage of grants, depending on achievement of specific performance targets | 230.00% |  |

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