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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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.

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Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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The amount of cash paid for commissions during the current period.

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Goodwill and Other Intangibles - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended								12 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated impairment losses			$ 11,600				$ 11,600				$ 11,600	$ 11,600
Goodwill	$ 29,835	$ 29,835	29,835	$ 29,835	$ 29,835	$ 29,835	29,835	$ 29,835	$ 29,835	$ 29,835	29,835	29,835
Asset impairment charges			$ 2,234	$ 0	$ 0	$ 0	$ 1	$ 0	$ 0	$ 0	2,234	1
Asset Purchase Agreement - Alternative Fuel Supply Technology
Finite-Lived Intangible Assets [Line Items]
Guaranteed payment	1,200	$ 1,200									1,000
Additional payment for future sales milestones	3,300
Asset Purchase Agreement - Alternative Fuel Supply Technology \| Intellectual Property
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired	1,500										$ 1,000	$ 500
Asset Purchase Agreement - Alternative Fuel Supply Technology \| Developed technology
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired	$ 2,700
Useful life	3 years