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Employee Benefit Plans (Tables) | 12 Months Ended
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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 year |  | Average price paid per share |  | Weighted-average fair value of discount under the ESPP (1)
As of December 31, 2018 |  | 33,300 |  |  | $ | 134.31 |  |  | $ | 16.79 |
As of December 31, 2019 |  | 38,464 |  |  | $ | 133.54 |  |  | $ | 23.51 |
As of December 31, 2020 |  | 56,866 |  |  | $ | 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:
| Options |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Life (years) |  | Aggregate Intrinsic Value (thousands)
Outstanding at December 31, 2017 | 27,575 |  |  | $ | 97.88 |  |  | 0.72 |  | $ | 1,568 |
Exercised | (17,838) |  |  | 92.04 |  |  |  |  |
Outstanding at December 31, 2018 | 9,737 |  |  | $ | 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:
| Shares |  | Weighted Average Grant Date Fair Value Per Share
Non-vested at December 31, 2017 | 192,890 |  |  | $ | 153.32 |
Granted | 102,842 |  |  | 155.02 |
Vested | (85,410) |  |  | 153.85 |
Forfeited | (14,128) |  |  | 154.65 |
Non-vested at December 31, 2018 | 196,194 |  |  | $ | 153.88 |
Granted | 218,477 |  |  | 143.72 |
Vested | (104,816) |  |  | 152.07 |
Forfeited | (15,047) |  |  | 148.97 |
Non-vested at December 31, 2019 | 294,808 |  |  | $ | 147.25 |
Granted | 267,169 |  |  | 137.80 |
Vested | (291,303) |  |  | 147.58 |
Forfeited | (5,147) |  |  | 145.82 |
Non-vested at December 31, 2020 | 265,527 |  |  | $ | 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):
| 2020 |  | 2019 |  | 2018
Weighted-average volatility | — | % |  | — | % |  | 35.0 | %
Expected term (in years) | N/A |  | N/A |  | 0.8
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 SARs |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Term (years) |  | Aggregate Intrinsic Value (thousands)
Balance at December 31, 2017 |  | 80,464 |  |  | $ | 155.13 |  |  |  |  |
Forfeited |  | (12,890) |  |  | 181.47 |  |  |  |  |
Exercised |  | (13,642) |  |  | 98.37 |  |  |  |  |
Balance at December 31, 2018 |  | 53,932 |  |  | $ | 163.19 |  |  |  |  |
Exercised |  | (9,886) |  |  | 135.53 |  |  |  |  |
Balance at December 31, 2019 |  | 44,046 |  |  | $ | 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 |  | $ | — |

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- Definition Tabular disclosure of the changes in outstanding cash settled stock appreciation rights. + References No definition available. + Details
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- Definition Tabular disclosure of input and valuation technique used to measure fair value and change in valuation approach and technique for each separate class of asset and liability measured on recurring and nonrecurring basis. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition Tabular disclosure of the changes in outstanding nonvested restricted stock units. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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