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Share-Based Compensation and Benefit Plans (Tables) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Share-based Payment Arrangement, Noncash Expense [Abstract] |  |
Summary Of Share Incentive Plans | A summary of activity under our share incentive plans for the year ended December 31, 2020 is shown below:
|  | Nonvested Share Awards Outstanding |  | Weighted Average Exercise / Grant Price
Nonvested share awards outstanding at December 31, 2019 |  | 264,654 |  |  | $ | 90.44 |
Granted |  | 181,298 |  |  | 113.46 |
Exercised/Vested |  | (196,464) |  |  | 95.12 |
Forfeited |  | (9,760) |  |  | 103.61 |
Total nonvested share awards outstanding at December 31, 2020 |  | 239,728 |  |  | $ | 103.48 |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] |  | The weighted-average fair value of reload stock options granted during the year ended December 31, 2018 and the weighted-average assumptions for such grants were as follows:
|  |  |  |  |  | Year Ended December 31, 2018
Weighted average fair value of options granted |  |  |  |  |  | $4.11
Expected volatility |  |  |  |  |  | 15.1%
Risk-free interest rate |  |  |  |  |  | 2%
Expected dividend yield |  |  |  |  |  | 3.3%
Expected life |  |  |  |  |  | 1 year
Schedule of Share-based Compensation, Employee Stock Purchase Plan, Activity [Table Text Block] | The following table presents information related to our ESPP:
| 2020 |  | 2019 |  | 2018
Shares purchased | 22,496 |  |  | 22,032 |  |  | 15,330 |
Weighted average fair value of shares purchased | $ | 95.97 |  |  | $ | 105.93 |  |  | $ | 90.93 |
Expense recorded (in millions) | $ | 0.3 |  |  | $ | 0.4 |  |  | $ | 0.2 |
Temporary Equity [Table Text Block] | The following table summarizes the eligible share award activity for the year ended December 31, 2018, prior to the amendment effective January 1, 2019:
(in thousands) |  |  |  | 2018
Temporary equity: |  |  |  |
Balance at beginning of period |  |  |  | $ | 77,230 |
Change in classification |  |  |  | 16,407 |
Change in redemption value |  |  |  | (669) |
Diversification of share awards (429 shares during December 31, 2018) |  |  |  | (40,294) |
Balance at December 31, 2018 |  |  |  | $ | 52,674 |

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- Definition Tabular disclosure of activity for award under share-based payment arrangement. Includes, but is not limited to, outstanding award at beginning and end of year, granted, exercised, forfeited, 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 (e) -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 (c)(2) -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 (d) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of employee stock purchase plan activity. + 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 (g) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + 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 (f)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of temporary equity. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. If convertible, the issuer does not control the actions or events necessary to issue the maximum number of shares that could be required to be delivered under the conversion option if the holder exercises the option to convert the stock to another class of equity. If the security is a warrant or a rights issue, the warrant or rights issue is considered to be temporary equity if the issuer cannot demonstrate that it would be able to deliver upon the exercise of the option by the holder in all cases. Includes stock with put option held by ESOP and stock redeemable by holder only in the event of a change in control of the issuer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (27) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=65877616&loc=d3e177068-122764 + Details
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