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Acquisitions (Narrative) (Detail) (USD $)
 3 Months Ended 9 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended 9 Months Ended 9 Months Ended 9 Months Ended 6 Months Ended 9 Months Ended
Sep. 30, 2012
Mar. 31, 2012
Entity
Sep. 30, 2011
Entity
Sep. 30, 2012
Dec. 31, 2011
Entity
Sep. 30, 2012
Electric Power Division [Member]
Dec. 31, 2011
Electric Power Division [Member]
Mar. 31, 2012
Electric Power Division [Member]
Canada [Member]
Entity
Sep. 30, 2011
Electric Power Division [Member]
Canada [Member]
Entity
Dec. 31, 2011
Electric Power Division [Member]
Canada [Member]
Entity
Jun. 30, 2012
Electric Power Division [Member]
United States [Member]
Entity
Mar. 31, 2012
Electric Power Division [Member]
United States [Member]
Entity
Dec. 31, 2011
Electric Power Division [Member]
United States [Member]
Entity
Sep. 30, 2012
Natural Gas And Pipeline Division [Member]
Dec. 31, 2011
Natural Gas And Pipeline Division [Member]
Mar. 31, 2012
Natural Gas And Pipeline Division [Member]
United States [Member]
Entity
Sep. 30, 2011
Natural Gas And Pipeline Division [Member]
Australia [Member]
Entity
Dec. 31, 2011
Natural Gas And Pipeline Division [Member]
Australia [Member]
Entity
Sep. 30, 2012
Service Electric [Member]
Apr. 12, 2012
Service Electric [Member]
Sep. 30, 2012
Crux [Member]
Jan. 09, 2012
Crux [Member]
Sep. 30, 2012
Microline [Member]
Jan. 04, 2012
Microline [Member]
Dec. 31, 2011
Third and fourth quarters 2011 acquisitions
Sep. 30, 2012
Phasor [Member]
Mar. 02, 2012
Phasor [Member]
Business Acquisition [Line Items]                                                      
Cash paid for acquisition                                       $ 20,300,000   $ 27,500,000   $ 6,800,000 $ 80,800,000   $ 2,900,000
Number of shares issued for acquisition of company                                     662,426   856,105   320,619   1,939,813 87,963  
Common stock value 37,291,000     37,291,000 32,368,000                             12,500,000   16,700,000   6,400,000 32,400,000   1,700,000
Payment of cash to repay acquiree debt                                       5,900,000   4,200,000   900,000 3,400,000    
Number of business acquisitions completed   3 4   5     1 3 3 1 1 1     1 1 1                  
Accounts receivables with fair value 15,300,000     15,300,000 16,100,000                                            
Goodwill 64,779,000     64,779,000 43,758,000 57,500,000 34,900,000             7,300,000 8,900,000                        
Goodwill expected to be deductible for income tax 52,900,000     52,900,000 13,100,000                                            
Revenues contributed by acquisitions 35,500,000     60,200,000                                              
Income before income taxes contributed by acquisitions $ (700,000)     $ (100,000)