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Segment Reporting - Schedule of Revenue Information from Products and Service Lines (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue from External Customer [Line Items]                      
Revenues $ 556,910 $ 519,878 $ 504,520 $ 542,208 $ 501,235 $ 506,000 $ 471,231 $ 416,454 $ 2,123,516 $ 1,894,920 $ 1,799,889
Nuclear Power Group                      
Revenue from External Customer [Line Items]                      
Revenues                 371,269 352,640 365,911
Nuclear Services Group                      
Revenue from External Customer [Line Items]                      
Revenues                 136,493 131,339 122,438
Nuclear Operations Group                      
Revenue from External Customer [Line Items]                      
Revenues                 1,646,257 1,428,587 1,319,170
Reportable Segments                      
Revenue from External Customer [Line Items]                      
Revenues                 2,154,019 1,912,566 1,807,519
Reportable Segments | Nuclear Power Group                      
Revenue from External Customer [Line Items]                      
Revenues                 371,269 352,640 365,911
Reportable Segments | Nuclear Power Group | Nuclear Services and Engineering                      
Revenue from External Customer [Line Items]                      
Revenues                 120,684 109,168 164,283
Reportable Segments | Nuclear Power Group | Nuclear Services and Engineering                      
Revenue from External Customer [Line Items]                      
Revenues                 250,585 243,472 201,628
Reportable Segments | Nuclear Services Group                      
Revenue from External Customer [Line Items]                      
Revenues                 136,493 131,339 122,438
Reportable Segments | Nuclear Services Group | Nuclear Environmental Services                      
Revenue from External Customer [Line Items]                      
Revenues                 94,320 102,726 96,566
Reportable Segments | Nuclear Services Group | Nuclear Services and Advanced Reactor Design and Engineering                      
Revenue from External Customer [Line Items]                      
Revenues                 42,173 28,613 25,872
Reportable Segments | Nuclear Operations Group                      
Revenue from External Customer [Line Items]                      
Revenues                 1,646,257 1,428,587 1,319,170
Reportable Segments | Nuclear Operations Group | Government Programs                      
Revenue from External Customer [Line Items]                      
Revenues                 1,540,256 1,368,555 1,311,886
Reportable Segments | Nuclear Operations Group | Commercial Operations                      
Revenue from External Customer [Line Items]                      
Revenues                 106,001 60,032 7,284
Other                      
Revenue from External Customer [Line Items]                      
Revenues                 $ (30,503) $ (17,646) $ (7,630)