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Intangible Assets - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2019 | Mar. 31, 2018 | Mar. 31, 2017
Finite Lived Intangible Assets [Line Items] |  |  |  |  |  |
Intangible amortization expense |  |  |  | $ 237 | $ 236 | $ 234
Impairment losses, intangible assets |  |  |  | 12,800 | 9,100 |
Impairment losses, goodwill | $ 1,222 |  | $ 5,716 |  |  |
Impairment of goodwill and intangible assets | 6,449 | $ 0 | 14,816 | 6,449 | 14,816 |
Goodwill | 0 |  |  | 0 | 1,222 |
Permits [Member] |  |  |  |  |  |
Finite Lived Intangible Assets [Line Items] |  |  |  |  |  |
Impairment losses, intangible assets | 1,700 |  | 8,600 | 10,300 | 8,600 |
Tradename [Member] |  |  |  |  |  |
Finite Lived Intangible Assets [Line Items] |  |  |  |  |  |
Impairment losses, intangible assets | 2,000 |  | $ 500 | 2,500 | 500 |
Customer Relationships [Member] |  |  |  |  |  |
Finite Lived Intangible Assets [Line Items] |  |  |  |  |  |
Intangible amortization expense |  |  |  | $ 180 | $ 180 | $ 180
Impairment losses, intangible assets | 1,208 |  |  |  |  |
Other Long-Lived Assets [Member] |  |  |  |  |  |
Finite Lived Intangible Assets [Line Items] |  |  |  |  |  |
Impairment losses, intangible assets | $ 319 |  |  |  |  |

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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 + Details
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