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Stock-based Compensation Expense (Tables) | 12 Months Ended
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Dec. 31, 2020
Share-based Payment Arrangement, Noncash Expense [Abstract] |
Fixed-Price Stock Options | The following table summarizes information about the Company’s fixed-price stock options as of and for the year ended December 31, 2020:
| Shares |  | Weighted-Average Exercise Price |  | Weighted-Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2019 | 1,244,531 |  |  | $ | 65.67 |  |  | 4.2 |  | $ | 14,593 |
Granted | 76,221 |  |  | 81.85 |  |  |  |  |
Exercised | (682,068) |  |  | 59.28 |  |  |  |  |
Forfeited | (37,843) |  |  | 94.03 |  |  |  |  |
Expired | (1,000) |  |  | 41.94 |  |  |  |  |
Outstanding at December 31, 2020 | 599,841 |  |  | $ | 73.24 |  |  | 5.6 |  | $ | 44,554 |
Exercisable at December 31, 2020 | 416,289 |  |  | $ | 64.06 |  |  | 4.4 |  | $ | 34,742 |
Weighted-Average Assumptions used to Estimate Fair Value of Each Option Granted | The fair value of each option granted during the years ended December 31, 2020, 2019 and 2018 was estimated on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Dividend yield | 1.69 | % |  | 1.58 | % |  | 1.44 | %
Volatility | 32.65 | % |  | 32.50 | % |  | 32.48 | %
Average expected life (years) | 6 |  | 6 |  | 6
Risk-free interest rate | 1.13 | % |  | 2.81 | % |  | 3.06 | %
Fair value of options granted | $ | 22.14 |  |  | $ | 27.71 |  |  | $ | 37.35 |
The calculation used the following weighted-average assumptions:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Volatility | 33.66 | % |  | 30.11 | % |  | 29.92 | %
Risk-free interest rate | 0.85 | % |  | 2.43 | % |  | 2.36 | %
Activity in Performance Unit Awards | The following table summarizes activity in performance unit awards as of and for the year ended December 31, 2020:
| Shares |  | Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period | 259,733 |  |  | $ | 115.69 |
Granted | 87,124 |  |  | 99.44 |
Vested | (41,749) |  |  | 78.03 |
Forfeited | (78,300) |  |  | 115.23 |
Nonvested, end of period | 226,808 |  |  | 116.54 |
Activity in Non-Performance Based Restricted Stock Awards | The following table summarizes activity in non-performance based restricted stock and restricted stock unit awards as of and for the year ended December 31, 2020:
| Shares |  | Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period | 272,560 |  |  | $ | 85.98 |
Granted | 185,261 |  |  | 71.70 |
Vested | (107,215) |  |  | 80.59 |
Forfeited | (23,862) |  |  | 89.31 |
Nonvested, end of period | 326,744 |  |  | 79.48 |

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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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- 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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