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Share-Based Payments - Share-based Payments - Non-Employee Directors RSUs (Details) - Restricted Share Units - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
RSUs granted, number of shares (in shares)	221,943	174,711
Non-Employee Directors
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
RSUs granted, number of shares (in shares)	29,025	24,456
Board of Directors Chairman
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
RSUs granted, number of shares (in shares)	12,900	10,962

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The number of grants made 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

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