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Stock-based Compensation Expense (Tables) | 12 Months Ended
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Dec. 31, 2018
Share-based Compensation [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, 2018:
| Shares |  | Weighted-Average Exercise Price |  | Weighted-Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2017 | 1,401,718 |  |  | $ | 56.10 |  |  | 5.1 |  | $ | 100,632 |
Granted | 63,259 |  |  | 118.75 |  |  |  |  |
Exercised | (94,031 | ) |  | 38.64 |  |  |  |  |
Forfeited | (54,213 | ) |  | 75.72 |  |  |  |  |
Outstanding at December 31, 2018 | 1,316,733 |  |  | $ | 59.55 |  |  | 4.3 |  | $ | 26,438 |
Exercisable at December 31, 2018 | 911,011 |  |  | $ | 54.28 |  |  | 3.2 |  | $ | 20,872 |
Weighted-Average Assumptions used to Estimate Fair Value of Each Option Granted | The calculation used the following weighted-average assumptions:
| Year Ended December 31,
| 2018 |  | 2017 |  | 2016
Volatility | 29.92 | % |  | 30.34 | % |  | 27.69 | %
Risk-free interest rate | 2.36 | % |  | 1.34 | % |  | 0.91 | %
The fair value of each option granted during the years ended December 31, 2018, 2017 and 2016 was estimated on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions:
| Year Ended December 31,
| 2018 |  | 2017 |  | 2016
Dividend yield | 1.44 | % |  | 1.56 | % |  | 1.84 | %
Volatility | 32.48 | % |  | 32.70 | % |  | 33.08 | %
Average expected life (years) | 6 |  |  | 6 |  |  | 6 |
Risk-free interest rate | 3.06 | % |  | 2.51 | % |  | 1.96 | %
Fair value of options granted | $ | 37.35 |  |  | $ | 27.99 |  |  | $ | 16.06 |
Activity in Performance Unit Awards | The following table summarizes activity in performance unit awards as of and for the year ended December 31, 2018:
| Shares |  | Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period | 433,003 |  |  | $ | 75.55 |
Granted | 70,274 |  |  | 155.65 |
Vested | (164,303 | ) |  | 61.72 |
Forfeited | (21,537 | ) |  | 120.49 |
Nonvested, end of period | 317,437 |  |  | 97.39 |
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, 2018:
| Shares |  | Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period | 223,338 |  |  | $ | 71.95 |
Granted | 104,513 |  |  | 104.36 |
Vested | (48,178 | ) |  | 66.93 |
Forfeited | (22,155 | ) |  | 78.98 |
Nonvested, end of period | 257,518 |  |  | 85.44 |

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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=109197908&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=109197908&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=109197908&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=109197908&loc=d3e5070-113901 + Details
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