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Stock-Based Compensation - Summary of Stock-Based Compensation Grants (Parenthetical) (Detail) - shares | 3 Months Ended | 12 Months Ended
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Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Performance Based Restricted Stock Unit Target [Member] |  |  |  |
Stock Awards Granted | 32,462 |  |  |
Performance Based Restricted Stock Unit Adjustment [Member] |  |  |  |
Stock Awards Granted |  | 5,588 | 14,900 | 27,154

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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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