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Stock-based Compensation Expense Activity in Performance Unit Awards (Details) - Performance Unit Awards
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Shares  
Nonvested, beginning of period (in shares) | shares 433,003
Granted (in shares) | shares 70,274
Vested (in shares) | shares (164,303)
Forfeited (in shares) | shares (21,537)
Nonvested, end of period (in shares) | shares 317,437
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period (in dollars per share) | $ / shares $ 75.55
Granted (in dollars per share) | $ / shares 155.65
Vested (in dollars per share) | $ / shares 61.72
Forfeited (in dollars per share) | $ / shares 120.49
Nonvested, end of period (in dollars per share) | $ / shares $ 97.39