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Stock-Based Compensation - Shares of Restricted Stock Granted under 2010 Plan (Details) - Plan 2010 - Restricted Stock - shares | 3 Months Ended | 12 Months Ended
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Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Shares | 75,804 |  |  |  |
Remaining Term (In Years) | 2 years 1 month 6 days |  |  |  |
2015 | Five Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) |  |  |  |  | 5 years
Shares | 12,023 |  |  |  |
Remaining Term (In Years) |  |  |  |  | 27 days
2016 | Five Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) |  |  |  | 5 years |
Shares | 15,015 |  |  |  |
Remaining Term (In Years) |  |  |  | 1 year 9 months 18 days |
2017 | Five Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) |  |  | 5 years |  |
Shares | 9,972 |  |  |  |
Remaining Term (In Years) |  |  | 2 years 9 months 18 days |  |
2017 | Three Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) |  |  | 3 years |  |
Shares | 4,902 |  |  |  |
Remaining Term (In Years) |  |  | 27 days |  |
2018 | Five Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) |  | 5 years |  |  |
Shares | 9,655 |  |  |  |
Remaining Term (In Years) |  | 3 years 9 months 18 days |  |  |
2018 | Three Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) |  | 3 years |  |  |
Shares | 2,150 |  |  |  |
Remaining Term (In Years) |  | 1 year 10 months 24 days |  |  |
2018 | Two Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) |  | 2 years |  |  |
Shares | 932 |  |  |  |
Remaining Term (In Years) |  | 24 days |  |  |
2018 | Four Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) |  | 4 years |  |  |
Shares | 2,068 |  |  |  |
Remaining Term (In Years) |  | 2 years 9 months 18 days |  |  |
2019 | Three Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) | 3 years |  |  |  |
Shares | 16,254 |  |  |  |
Remaining Term (In Years) | 2 years 9 months 18 days |  |  |  |
2019 | One Year |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) | 1 year |  |  |  |
Shares | 1,349 |  |  |  |
Remaining Term (In Years) | 24 days |  |  |  |
2019 | Two Years |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting Term (In Years) | 2 years |  |  |  |
Shares | 1,484 |  |  |  |
Remaining Term (In Years) | 1 year 9 months 18 days |  |  |  |

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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 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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