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Stock-based Compensation Expense (Tables) | 12 Months Ended
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Dec. 31, 2017
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, 2017:
| Shares |  | Weighted-Average Exercise Price |  | Weighted-Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2016 | 1,542,244 |  |  | $ | 51.85 |  |  | 5.5 |  | $ | 52,798 |
Granted | 82,204 |  |  | 92.93 |  |  |  |  |
Exercised | (210,432 | ) |  | 39.15 |  |  |  |  |
Forfeited | (12,298 | ) |  | 59.05 |  |  |  |  |
Outstanding at December 31, 2017 | 1,401,718 |  |  | $ | 56.10 |  |  | 5.1 |  | $ | 100,632 |
Exercisable at December 31, 2017 | 768,436 |  |  | $ | 50.35 |  |  | 3.3 |  | $ | 59,585 |
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, 2017, 2016 and 2015 was estimated on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions:
| Year Ended December 31,
| 2017 |  | 2016 |  | 2015
Dividend yield | 1.56 | % |  | 1.84 | % |  | 1.80 | %
Volatility | 32.70 | % |  | 33.08 | % |  | 32.92 | %
Average expected life (years) | 6 |  |  | 6 |  |  | 6 |
Risk-free interest rate | 2.51 | % |  | 1.96 | % |  | 2.17 | %
Fair value of options granted | $ | 27.99 |  |  | $ | 16.06 |  |  | $ | 16.04 |
Activity in Performance Unit Awards | The following table summarizes activity in performance unit awards as of and for the year ended December 31, 2017:
| Shares |  | Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period | 494,691 |  |  | $ | 66.42 |
Granted | 82,396 |  |  | 116.43 |
Vested | (131,374 | ) |  | 67.59 |
Forfeited | (12,710 | ) |  | 67.36 |
Nonvested, end of period | 433,003 |  |  | 75.55 |
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, 2017:
| Shares |  | Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning of period | 180,656 |  |  | $ | 57.99 |
Granted | 84,528 |  |  | 97.49 |
Vested | (32,675 | ) |  | 63.45 |
Forfeited | (9,171 | ) |  | 60.17 |
Nonvested, end of period | 223,338 |  |  | 71.95 |

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- Definition Tabular disclosure of the changes in outstanding nonvested restricted stock units. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=96867065&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=96867065&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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