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Basis of Presentation and Significant Accounting Policies - Changes in Carrying Amount of Intangible Assets (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Balance at beginning of period $ 191,392 $ 228,676 $ 110,405
Acquisitions (Note 2) 8,180 0 139,257
Amortization expense (8,824) (9,128) (11,490)
Reclassification of right-of-use assets 0 (33,866) 0
Translation and other 2,003 5,710 (9,496)
Balance at end of period 192,751 $ 191,392 $ 228,676
Nuclear Services Group      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Translation and other $ (600)