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Note 15 - Revenue 1 (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
Stockholders' Equity Attributable to Parent, Ending Balance	$ 269,626	$ 248,158	$ 218,590		$ 200,264
Increase (Decrease) in Revenue from Contract with Customers, Including Assessed Tax	2,200	(1,200)	(200)
Contract with Customer, Liability, Revenue Recognized	12,300	$ 3,700	2,600
Revenue, Remaining Performance Obligation, Amount	$ 167,000
Ameron Water Transmission Group, LLC [Member]
Contract with Customer, Asset, Increase (Decrease) for Contract Acquired in Business Combination			12,000
Contract with Customer, Liability, Increase (Decrease) for Contract Acquired in Business Combination			$ 100
Revenue Benchmark [Member] | Customer Concentration Risk [Member]
Number of Major Customers	1	1	0
Concentration Risk, Percentage	16.00%	23.00%
Retained Earnings [Member]
Stockholders' Equity Attributable to Parent, Ending Balance	$ 148,381	$ 129,331	$ 101,194		81,757
Accounting Standards Update 2014-09 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Stockholders' Equity Attributable to Parent, Ending Balance					(875)
Accounting Standards Update 2014-09 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings [Member]
Stockholders' Equity Attributable to Parent, Ending Balance				$ (900)	$ (875)

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The increase (decrease) during the period of the amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount of increase (decrease) in right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, from business combination.

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Amount of increase (decrease) in obligation to transfer good or service to customer for which consideration from customer has been received or is due, from business combination.

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Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

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Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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