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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

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