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Stock-Based Compensation - Summary of Restricted Stock, Stock Grants, Stock Units and Performance Shares (Details) - $ / shares | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Restricted Stock Units, Stock Grants, and Stock Units |  |  |
Stocks granted and the weighted average fair value |  |  |
Units granted (in shares) | 161,963 | 141,811 | 152,651
Grant date fair value (in dollars per share) | $ 72.60 | $ 67.34 | $ 64.12
Number of granted awards to be settled in cash (in shares) | 67,599 | 43,952 | 45,104
Performance Shares |  |  |
Stocks granted and the weighted average fair value |  |  |
Units granted (in shares) | 147,706 | 166,666 | 151,430
Grant date fair value (in dollars per share) | $ 78.99 | $ 66.60 | $ 64.97

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- Definition The number of units granted during the period on other than stock (or unit) option plans. + References No definition available. + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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