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Share-Based Compensation Plans - Relative Total Shareholder Return Performance Unit Activity (Details) - RTSR - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Number of Non-vested Performance- Based Share Units      
Beginning balance (in shares) 142 62  
Granted (in shares) 58 80  
Vested (in shares) (24) 0  
Forfeited (in shares) 0 0  
Ending balance (in shares) 176 142 62
Weighted-Average Grant Date Fair Value Per Share      
Nonvested shares outstanding (in shares) $ 65.04 $ 63.02 $ 78.35
Granted (in dollars per share) 67.72 55.61 $ 101.13
Forfeited (in dollars per share) 0 0  
Vested (in dollars per share) $ 62.73 $ 0  
Weighted- Average Remaining Term in Years [1] 1 year 6 months 1 year 6 months  
Aggregate Intrinsic Value $ 7.9 $ 7.3  
Fair value of shares vested in period $ 1.5 $ 0.0 $ 0.0
[1] * Midpoint used in calculation.