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9. Share-based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2017
Share-based Compensation [Abstract] |
Stock Options Activity | Stock Options The following table summarizes stock option activity:
|  | Shares |  | Weighted- Average Exercise Price |  | Weighted- Average Remaining Contractual Term (in years) |  | Aggregate Intrinsic Value (in millions)
Outstanding, December 31, 2016 |  | 449,570 |  |  | $ | 106.52 |  |  |  |  |
Granted |  | 44,000 |  |  | $ | 215.98 |  |  |  |  |
Exercised |  | (78,890 | ) |  | $ | 89.41 |  |  |  |  |
Forfeited/expired |  | (7,200 | ) |  | $ | 177.99 |  |  |  |  |
Outstanding, December 31, 2017 |  | 407,480 |  |  | $ | 120.39 |  |  | 5.09 |  | $ | 48.2 |
Unvested, December 31, 2017 |  | 125,720 |  |  | $ | 164.84 |  |  | 8.29 |  | $ | 9.3 |
Exercisable, December 31, 2017 |  | 281,760 |  |  | $ | 100.56 |  |  | 3.66 |  | $ | 38.9 |
Stock Options Valuation Assumptions | The weighted-average fair value of stock options granted was estimated using a Black-Scholes option-pricing model with the following weighted-average assumptions:
|  | Year Ended December 31,
|  | 2017 |  | 2016 |  | 2015
Expected volatility |  | 20 | % |  | 21 | % |  | 23 | %
Risk-free interest rate |  | 1.87 | % |  | 1.35 | % |  | 1.90 | %
Expected life (in years) |  | 7.2 |  |  | 7.4 |  |  | 7.7 |
Expected dividend |  | — |  |  | — |  |  | — |
Weighted-average fair value of options granted |  | $ | 58.65 |  |  | $ | 42.40 |  |  | $ | 42.74 |
Resticted Stock Activity | The following table summarizes restricted stock unit activity:
|  | Restricted Stock Units |  | Weighted- Average Grant-Date Fair Value |  | Weighted-Average Remaining Contractual Term (in years) |  | Aggregate Intrinsic Value (in millions)
Outstanding, December 31, 2016 |  | 448,822 |  |  | $ | 141.09 |  |  |  |  |
Granted |  | 192,860 |  |  | $ | 215.98 |  |  |  |  |
Vested |  | (127,189 | ) |  | $ | 133.54 |  |  |  |  |
Forfeited |  | (41,263 | ) |  | $ | 151.36 |  |  |  |  |
Outstanding, December 31, 2017 |  | 473,230 |  |  | $ | 172.75 |  |  | 2.14 |  | $ | 112.9 |
Employee Stock Purchase Plan, Valuation Assumptions | The fair value of the employees’ purchase rights under the 2011 ESPP was estimated using a Black-Scholes model with the following weighted-average assumptions:
| Year Ended December 31,
| 2017 |  |  |  | 2016 |  |  |  | 2015
Expected volatility | 19 | % |  |  |  | 20 | % |  |  |  | 18 | %
Risk-free interest rate | 0.83 | % |  |  |  | 0.26 | % |  |  |  | 0.02 | %
Expected life (in years) | 0.24 |  |  |  |  | 0.25 |  |  |  |  | 0.25 |
Expected dividend | — |  |  |  |  | — |  |  |  |  | — |
Weighted-average fair value |  |  |  |  |  |  |  |  |
of purchase rights | $ | 38.86 |  |  |  |  | $ | 27.36 |  |  |  |  | $ | 25.08 |

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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of employee stock purchase plans, including, but not limited to: (a) expected term, (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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- 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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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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- Definition Tabular disclosure of the weighted-average grant-date fair value of equity options or other equity instruments granted during the year. + 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)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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