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Stock-based Compensation Expense Activity in Performance Unit Awards (Details) - Performance Unit Awards
12 Months Ended
Dec. 31, 2020
$ / shares
shares
Shares  
Nonvested, beginning of period (in shares) | shares 259,733
Granted (in shares) | shares 87,124
Vested (in shares) | shares (41,749)
Forfeited (in shares) | shares (78,300)
Nonvested, end of period (in shares) | shares 226,808
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period (in dollars per share) | $ / shares $ 115.69
Granted (in dollars per share) | $ / shares 99.44
Vested (in dollars per share) | $ / shares 78.03
Forfeited (in dollars per share) | $ / shares 115.23
Nonvested, end of period (in dollars per share) | $ / shares $ 116.54