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Employee Stock and Incentive Plans - Performance Shares and Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Feb. 28, 2019 | Jan. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Performance Shares [Member] |  |  |  |  |  |
Stock-based Compensation [Line Items] |  |  |  |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition |  |  |  | 1 year 8 months |  |
Number of Shares [Rollforward] |  |  |  |  |  |
As of January 1 |  |  | 129,693 | 127,898 | 127,580 | 119,540
Granted | [1] |  |  | 66,557 | 50,729 | 57,189
Awarded |  |  |  | (58,293) | 0 | 0
Unearned Grant Award |  |  |  | 0 | (40,801) | (42,126)
Forfeited |  |  |  | (6,469) | (9,610) | (7,023)
As of December 31 |  |  |  | 129,693 | 127,898 | 127,580
Performance Period (Years) |  |  |  | 3 years |  |
Weighted-Average Grant Date Fair Value [Abstract] |  |  |  |  |  |
As of January 1 |  |  | $ 66.12 | $ 58.23 | $ 52.56 | $ 52.72
Granted | [1] |  |  | 76.42 | 62.90 | 52.43
Awarded |  |  |  | 59.82 | 0.00 | 0.00
Unearned Grant Award |  |  |  | 0.00 | 46.27 | 52.70
Forfeited |  |  |  | 72.99 | 58.29 | 53.45
As of December 31 |  |  |  | $ 66.12 | $ 58.23 | $ 52.56
Restricted Stock Units [Member] |  |  |  |  |  |
Stock-based Compensation [Line Items] |  |  |  |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition |  |  |  | 1 year 7 months |  |
Number of Shares [Rollforward] |  |  |  |  |  |
As of January 1 |  |  | 49,771 | 55,248 | 54,728 | 57,694
Granted | [2] |  |  | 16,573 | 21,241 | 20,351
Awarded |  |  |  | (18,881) | (17,281) | (19,661)
Forfeited |  |  |  | (3,169) | (3,440) | (3,656)
As of December 31 |  |  |  | 49,771 | 55,248 | 54,728
Performance Period (Years) |  |  |  | 3 years |  |
Weighted-Average Grant Date Fair Value [Abstract] |  |  |  |  |  |
As of January 1 |  |  | $ 60.74 | $ 56.18 | $ 51.79 | $ 49.86
Granted | [2] |  |  | 71.11 | 62.20 | 50.25
Awarded |  |  |  | 55.78 | 49.72 | 44.33
Forfeited |  |  |  | 64.92 | 56.00 | 52.87
As of December 31 |  |  |  | $ 60.74 | $ 56.18 | $ 51.79
Subsequent Event [Member] | Performance Shares [Member] |  |  |  |  |  |
Number of Shares [Rollforward] |  |  |  |  |  |
Granted |  |  | 31,843 |  |  |
Awarded |  | (73,532) |  |  |  |
Performance Period (Years) |  |  | 3 years |  |  |
Granted, Grant Date Fair Value |  |  | $ 2.3 |  |  |
Awarded, Grant Date Fair Value |  | $ 3.9 |  |  |  |
Subsequent Event [Member] | Restricted Stock Units [Member] |  |  |  |  |  |
Number of Shares [Rollforward] |  |  |  |  |  |
Granted |  |  | 12,924 |  |  |
Awarded |  | (14,307) |  |  |  |
Granted, Grant Date Fair Value |  |  | $ 1.0 |  |  |
Awarded, Grant Date Fair Value |  | $ 0.7 |  |  |  |
[1] | Shares granted include accrued dividends.
[2] | Shares granted include accrued dividends.

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- Definition The total grant date fair value of equity-based awards other than options, granted during the period. + References No definition available. + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(3) -URI http://asc.fasb.org/extlink&oid=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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