Note 2 - Accounting Policies (Details)	3 Months Ended			9 Months Ended		24 Months Ended			9 Months Ended			9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended		3 Months Ended				3 Months Ended
	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)	Sep. 30, 2013 EUR (€)	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2015 GBP (£)	Dec. 31, 2013 USD ($)	Sep. 30, 2014 Minimum [Member]	Sep. 30, 2014 Maximum [Member]	Sep. 30, 2013 Insituform Germany [Member]	Sep. 30, 2014 Europe [Member] USD ($)	Dec. 31, 2013 Europe [Member] USD ($)	Sep. 30, 2014 Asia Pacific [Member] USD ($)	Dec. 31, 2013 Asia Pacific [Member] USD ($)	Sep. 30, 2014 Bayou [Member] USD ($)	Sep. 30, 2014 Bayou [Member] USD ($)	Dec. 31, 2013 Bayou [Member] USD ($)	Sep. 30, 2014 Delta Double Jointing [Member]	Sep. 30, 2014 Bayou Coating [Member] USD ($)	Sep. 30, 2013 Bayou Coating [Member] USD ($)	Mar. 31, 2014 Bayou Coating [Member]	Dec. 31, 2013 Bayou Coating [Member]	Sep. 30, 2014 Bayou Delta [Member] USD ($)	Sep. 30, 2014 FRANCE USD ($)	Sep. 30, 2014 MALAYSIA USD ($)	Sep. 30, 2014 INDIA USD ($)
Note 2 - Accounting Policies [Line Items]
Tangible Asset Impairment Charges	$ 11,870,000	$ 1,100,000														$ 11,000,000								$ 0	$ 0	$ 0	$ 0
Fair Value Inputs, Discount Rate												14.00%		14.00%			13.50%
Fair Value Inputs, Long-term Revenue Growth Rate									2.50%	3.00%			3.00%		3.00%			3.00%
Compound Annual Growth Rate Assumption									2.00%	7.00%		5.00%		5.00%			5.00%
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount												9.30%		12.10%		10.20%	10.20%
PercentOfBookValue																						49.00%	49.00%
Sale of interests in German joint venture				9,065,000	0
Equity Method Investment, Other than Temporary Impairment																					2,700,000
Equity Method Investment Other Than Temporary Impairment Net Of Tax																					1,800,000
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	0	1,691,000		677,000	3,903,000															1,200,000
Proceeds from Equity Method Investment, Dividends or Distributions				684,000	4,027,000
Gain (Loss) on Sale of Equity Investments				(472,000)	0
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	(13,900,000)	2,100,000		(13,900,000)	2,100,000
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(100,000)	1,300,000		(100,000)	1,300,000
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(200,000)	200,000		(200,000)	200,000
Noncontrolling Interest, Ownership Percentage by Parent											50.00%								59.00%
Total consideration recorded		18,300,000	14,000,000	0	(18,300,000)
Gain (Loss) on Disposition of Business		11,300,000		0	11,771,000
Business Acquisition, Transaction Costs		500,000			500,000
Long-term Purchase Commitment, Amount (in Pounds)						3,600,000	2,300,000
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners																			41.00%	51.00%
Payments to Noncontrolling Interests				617,000	0
Equity Method Investment, Ownership Percentage									20.00%	50.00%
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	0			0				9,101,000
Goodwill	347,236,000			347,236,000				348,680,000				20,400,000		5,300,000		29,700,000	29,700,000
Impairment of Intangible Assets, Finite-lived	10,896,000	0		10,896,000	0
Reporting Unit, Change in Fair Value, Value													$ (8,200,000)		$ (18,000,000)			$ (48,000,000)
Reporting Unit, Change in Fair Value, Percent													(9.50%)		(36.40%)			(30.90%)