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Basis of Presentation and Significant Accounting Policies - Changes in Carrying Amount of Goodwill (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Goodwill [Roll Forward]      
Beginning balance $ 275,502 $ 274,082  
Goodwill, Acquired During Period 5,753    
Translation and other 2,453 1,420  
Ending balance 283,708 275,502 $ 274,082
Goodwill adjustment for business combination (3,200)    
Amortization of Debt Issuance Costs (2,540) (1,577) (4,062)
Additions (6,803) 0 (9,443)
Nuclear Operations Group      
Goodwill [Roll Forward]      
Beginning balance 110,939 110,939  
Goodwill, Acquired During Period 0    
Translation and other 0 0  
Ending balance 110,939 110,939 110,939
Nuclear Services Group      
Goodwill [Roll Forward]      
Beginning balance 119,563 118,143  
Goodwill, Acquired During Period 5,753    
Translation and other 2,453 1,420  
Ending balance 127,769 119,563 118,143
Nuclear Services Group      
Goodwill [Roll Forward]      
Beginning balance 45,000 45,000  
Goodwill, Acquired During Period 0    
Translation and other 0 0  
Ending balance $ 45,000 $ 45,000 $ 45,000