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Segment Information Segment Results Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Information [Line Items]                      
Warranty charge                     $ 29,400
Gross Profit $ 174,751 $ 165,318 $ 177,860 $ 157,225 $ 156,663 $ 170,749 $ 169,705 $ 163,203 $ 675,154 $ 660,320 $ 556,685
Earnings per common share - Basic $ 0.05 $ 0.66 $ 0.36 $ 0.41 $ 0.30 $ (0.26) $ 0.52 $ 0.27 $ 1.48 $ 0.83 $ 0.33
Earnings per common share - Diluted $ 0.05 $ 0.65 $ 0.36 $ 0.40 $ 0.30 $ (0.26) $ 0.52 $ 0.26 $ 1.45 $ 0.82 $ 0.33
Unusual or Infrequent Item, or Both, Insurance Proceeds     $ 8,000           $ 8,000    
Number of Operating Segments                 3 3 3
Electricity Operating Segment [Member]                      
Segment Reporting Information [Line Items]                      
Gross Profit                 $ 318,953 $ 282,677 $ 225,446
Water Operating Segment [Member]                      
Segment Reporting Information [Line Items]                      
Gross Profit                 $ 164,898 $ 172,580 145,680
Customers purchasing certain communication modules manufactured between July 2013 and December 2014 [Domain] | Water Operating Segment [Member] | Warranty Obligations [Member]                      
Segment Reporting Information [Line Items]                      
Warranty charge                     $ 29,400
Earnings per common share - Basic                     $ (0.47)
Earnings per common share - Diluted                     $ (0.47)
Sales [Member] | Customer A [Member] | Electricity Operating Segment [Member]                      
Segment Reporting Information [Line Items]                      
Percentage of total company revenue represented by one customer                 19.00% 12.00%  
Sales [Member] | Customer B [Member] | Electricity Operating Segment [Member]                      
Segment Reporting Information [Line Items]                      
Percentage of total company revenue represented by one customer                 11.00% 10.00%  
Sales [Member] | Customer C [Member] | Electricity Operating Segment [Member]                      
Segment Reporting Information [Line Items]                      
Percentage of total company revenue represented by one customer                 11.00%    
Sales [Member] | Threshold for Reporting Customer Concentration [Member]                      
Segment Reporting Information [Line Items]                      
Percentage of total company revenue represented by one customer                 10.00% 10.00% 10.00%
Insurance Recovery, net of tax [Member] | Water Operating Segment [Member]                      
Segment Reporting Information [Line Items]                      
Earnings per common share - Basic     $ 0.13           $ 0.13    
Earnings per common share - Diluted     $ 0.12           $ 0.12