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Stock-Based Compensation (Liability Classified Awards - Cash-settled Restricted Stock Units) (Details) - Cash-settled Restricted Stock Units
3 Months Ended
Mar. 31, 2019
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]  
Restricted stock, beginning of period (in shares) | shares 53,766
Restricted stock, granted (in shares) | shares 19,736
Restricted stock, released (in shares) | shares (317)
Restricted stock, forfeited (in shares) | shares (3,622)
Restricted stock, end of period (in shares) | shares 69,563
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]  
Weighted average grant date fair value, beginning balance (per share) | $ / shares $ 94.77
Weighted average fair value of restricted stock units granted (USD per share) | $ / shares 91.05
Weighted average fair value of restricted stock units released (in USD per share) | $ / shares 82.16
Weighted average grant date fair value, forfeited (per share) | $ / shares 98.96
Weighted average grant date fair value, ending balance (per share) | $ / shares $ 93.58