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Share-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Schedule of Recent Share-Based Compensation Expense | The following is a summary of share-based compensation expense for the years ended December 31, 2015, 2016 and 2017:
Year |  | Share-Based Compensation Expense |  | Tax Benefit
2015 |  | $ | 34.2 |  |  | $ | 13.2 |
2016 |  | 39.2 |  |  | 15.1 |
2017 |  | 40.4 |  |  | 13.6 |
Schedule of Transactions of the Company's Stock Options | The following table summarizes the transactions of the Company’s stock options:
| Stock Options |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Life (years)
Unexercised options outstanding—January 1, 2017 | 1.4 |  |  | $ | 108.53 |  |  |
Options granted | 0.0 |  |  | 168.60 |  |  |
Options exercised | (0.7 | ) |  | 97.54 |  |  |
Options forfeited | (0.1 | ) |  | 140.26 |  |  |
Unexercised options outstanding—December 31, 2017 | 0.6 |  |  | 122.04 |  |  | 3.9
Exercisable at December 31, 2017 | 0.2 |  |  | 115.74 |  |  | 1.7
Schedule of Assumptions Used to Determine Fair Value of Options Granted | The fair value of options granted was estimated using the Black-Scholes option pricing model. The weighted average fair value of options granted during the years ended December 31, 2015, 2016 and 2017 was $54.92, $39.02 and $48.05, per option, respectively, based on the weighted-average grant date assumptions stated below.
| For the Years Ended December 31,
| 2015 |  | 2016 |  | 2017
Dividend yield | 0.0 | % |  | 0.0 | % |  | 0.5 | %
Expected volatility(1) | 26.7 | % |  | 30.7 | % |  | 28.0 | %
Risk-free interest rate(2) | 1.5 | % |  | 1.6 | % |  | 2.1 | %
Expected life of options (in years)(3) | 5.0 |  |  | 5.7 |  |  | 5.7 |
Forfeiture rate | 0.0 | % |  | 0.0 | % |  | 0.0 | %
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(1) | Expected volatility is based on historical and implied volatility.
(2) | Risk-free interest rate is based on the U.S. Treasury yield curve in effect at the date of grant.
(3) | Expected life of options (in years) is based on the Company’s historical and expected exercise behavior.
Schedule of Transactions of the Company's Restricted Stock | The following table summarizes the transactions of the Company’s restricted stock:
| Restricted Stock |  | Weighted Average Grant Date Value
Unvested units—January 1, 2017 | 0.6 |  |  | $ | 168.84 |
Units granted | 0.2 |  |  | 152.99 |
Units vested | (0.4 | ) |  | 166.22 |
Units forfeited | (0.0 | ) |  | 170.39 |
Unvested units—December 31, 2017 | 0.4 |  |  | 162.32 |

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- Definition Tabular disclosure of the changes in outstanding nonvested shares. + 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 share-based compensation 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 (c)(2) -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 (d) -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 (e) -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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