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Share-based Payments - Summary of Restricted Share Units, Non-Employee Directors, under 2013 Share Incentive Plan (Details) - Restricted Share Units - $ / shares | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of shares outstanding, end of period | 439,576 |  |
2013 Share Incentive Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of shares outstanding, beginning of period | 530,340 | 504,234 | 519,112
Granted (in shares) | 200,020.707095856 | 328,550 | 287,852
Vested (in shares) | (245,703.707095856) | (246,489) | (266,944)
Forfeited (in shares) | (45,081) | (55,955) | (35,786)
Number of shares outstanding, end of period | 439,576 | 530,340 | 504,234
Number of shares outstanding, weighted average grant date fair value beginning of period (in dollars per share) | $ 39.28 | $ 40.45 | $ 35.82
Granted, weighted average grant date fair value (in dollars per share) | 49.20 | 37.63 | 39.68
Vested, weighted average grant date fair value (in dollars per share) | 39.63 | 34.26 | 34.26
Forfeited, weighted average grant date fair value (in dollars per share) | 41.84 | 39.51 | 39.59
Number of shares outstanding, weighted average grant date fair value end of period (in dollars per share) | $ 43.22 | $ 39.28 | $ 40.45
Non-Employee Directors | 2013 Share Incentive Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of shares outstanding, beginning of period | 5,171 | 6,636 | 6,806
Granted (in shares) | 31,122 | 35,408 | 39,774
Vested (in shares) | (29,624) | (33,769) | (39,944)
Forfeited (in shares) | (1,853) | (3,104) | 0
Number of shares outstanding, end of period | 4,816 | 5,171 | 6,636
Number of shares outstanding, weighted average grant date fair value beginning of period (in dollars per share) | $ 41.07 | $ 45.28 | $ 35.83
Granted, weighted average grant date fair value (in dollars per share) | 50.18 | 41.07 | 45.25
Vested, weighted average grant date fair value (in dollars per share) | 48.59 | 41.89 | 43.81
Forfeited, weighted average grant date fair value (in dollars per share) | 50.18 | 41.07 | 0.00
Number of shares outstanding, weighted average grant date fair value end of period (in dollars per share) | $ 50.19 | $ 41.07 | $ 45.28

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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + 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)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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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 + 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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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + 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)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or 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)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement. + 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)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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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. + Details
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