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Stock-based Compensation Expense Activity in Performance Unit Awards (Details) - Performance Unit Awards
12 Months Ended
Dec. 31, 2019
$ / shares
shares
Shares  
Nonvested, beginning of period (in shares) | shares 317,437
Granted (in shares) | shares 100,288
Vested (in shares) | shares (139,034)
Forfeited (in shares) | shares (18,958)
Nonvested, end of period (in shares) | shares 259,733
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period (in dollars per share) | $ / shares $ 97.39
Granted (in dollars per share) | $ / shares 107.68
Vested (in dollars per share) | $ / shares 66.93
Forfeited (in dollars per share) | $ / shares 124.45
Nonvested, end of period (in dollars per share) | $ / shares $ 115.69