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Employee Benefit Plans (Tables)
 12 Months Ended
Dec. 31, 2020
Employee Benefit Plans [Abstract]  
Schedule of Share-based Compensation, Employee Stock Purchase Plan, Activity [Table Text Block]
The following table provides information about the Company’s ESPP activity during 2020, 2019, and 2018:
Total number of shares purchased in yearAverage price paid per share
Weighted-average fair value of discount under the ESPP (1)
As of December 31, 201833,300 $134.31 $16.79 
As of December 31, 201938,464 $133.54 $23.51 
As of December 31, 202056,866 $90.63 $14.10 
(1) The weighted-average fair value of the discount under the ESPP granted is equal to a percentage discount from the market value of the common stock at the end of each semi-annual purchase period. The Company increased the discount from 10 percent to 15 percent for the second offering period of 2018. 15 percent is the maximum allowable discount under the ESPP.
Schedule of Share-based Compensation, Stock Options, Activity A summary of option activity as of December 31, 2020, 2019 and 2018, and changes during the years then ended, is presented below:
OptionsWeighted Average Exercise PriceWeighted Average Remaining Contractual Life (years)Aggregate Intrinsic Value (thousands)
Outstanding at December 31, 201727,575 $97.88 0.72$1,568 
Exercised(17,838)92.04 
Outstanding at December 31, 20189,737 $108.59 0.18$— 
Exercised(9,737)108.59 
Outstanding at December 31, 2019— $— 0.00$— 
Exercised— — 
Outstanding at December 31, 2020— $— 0.00$— 
Schedule of Nonvested Restricted Stock Units Activity A summary of the status of non-vested restricted stock grants during the years ended December 31, 2020, 2019 and 2018 is presented below:
SharesWeighted Average Grant Date Fair Value Per Share
Non-vested at December 31, 2017192,890 $153.32 
Granted102,842 155.02 
Vested(85,410)153.85 
Forfeited(14,128)154.65 
Non-vested at December 31, 2018196,194 $153.88 
Granted218,477 143.72 
Vested(104,816)152.07 
Forfeited(15,047)148.97 
Non-vested at December 31, 2019294,808 $147.25 
Granted267,169 137.80 
Vested(291,303)147.58 
Forfeited(5,147)145.82 
Non-vested at December 31, 2020265,527 $142.25 
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques
The following range of assumptions in the Black-Scholes pricing model was used to determine fair value as of December 31 of the years indicated below (all cash-settled SARs were fully vested as of December 31, 2019):

202020192018
Weighted-average volatility— %— %35.0 %
Expected term (in years)N/AN/A0.8
Risk-free interest rate— %— %2.6 %
Dividend yield— %— %1.68 %
Schedule Of Cash Settled Stock Appreciation Rights Activity
A summary of cash-settled SARs awards activity during the years ended December 31, 2020, 2019 and 2018 is presented below:

# of SARsWeighted Average Exercise PriceWeighted Average Remaining Contractual Term (years)Aggregate Intrinsic Value (thousands)
Balance at December 31, 201780,464 $155.13 
Forfeited(12,890)181.47 
Exercised(13,642)98.37 
Balance at December 31, 201853,932 $163.19 
Exercised(9,886)135.53 
Balance at December 31, 201944,046 $169.40 
Forfeited(29,046)181.47 
Exercised(15,000)186.85 
Balance at December 31, 2020— $— 0.00$— 
Vested at December 31, 2020— $— 0.00$— 
Exercisable at December 31, 2020— $— 0.00$—