Revenue Recognition - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 542,208	$ 416,454
Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues	284,866	245,760
Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues	195,810	107,509
Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues	48,489	42,388
Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues	19,292	22,637
Over time
Disaggregation of Revenue [Line Items]
Revenues	537,689	404,488
Point-in-time
Disaggregation of Revenue [Line Items]
Revenues	10,768	13,806
United States:
Disaggregation of Revenue [Line Items]
Revenues	467,320	340,607
United States: | Government
Disaggregation of Revenue [Line Items]
Revenues	425,662	321,754
United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	41,658	18,853
Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	75,487	68,901
Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	5,650	8,786
Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenues	423,775	304,801
Nuclear Operations Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues		245,487	$ 284,374
Nuclear Operations Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues		39,343	113,337
Nuclear Operations Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues		19,853	25,996
Nuclear Operations Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues		118	68
Nuclear Operations Group | Over time
Disaggregation of Revenue [Line Items]
Revenues		304,733	423,739
Nuclear Operations Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenues		68	36
Nuclear Power Group
Disaggregation of Revenue [Line Items]
Revenues	87,917	84,399
Nuclear Power Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues		273	492
Nuclear Power Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues		61,998	70,836
Nuclear Power Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues		0	0
Nuclear Power Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues		22,128	16,589
Nuclear Power Group | Over time
Disaggregation of Revenue [Line Items]
Revenues		70,661	77,185
Nuclear Power Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenues		13,738	10,732
Nuclear Services Group
Disaggregation of Revenue [Line Items]
Revenues	36,765	29,094
Nuclear Services Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues		0	0
Nuclear Services Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues		6,168	11,637
Nuclear Services Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues		22,535	22,493
Nuclear Services Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues		391	2,635
Nuclear Services Group | Over time
Disaggregation of Revenue [Line Items]
Revenues		29,094	36,765
Nuclear Services Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenues		0	0
Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	548,457	418,294
Operating Segments | Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenues	423,775	304,801
Operating Segments | Nuclear Operations Group | United States:
Disaggregation of Revenue [Line Items]
Revenues	421,907	302,641
Operating Segments | Nuclear Operations Group | United States: | Government
Disaggregation of Revenue [Line Items]
Revenues		297,303	401,066
Operating Segments | Nuclear Operations Group | United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues		5,338	20,841
Operating Segments | Nuclear Operations Group | Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues		0	0
Operating Segments | Nuclear Operations Group | Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues		2,160	1,868
Operating Segments | Nuclear Power Group
Disaggregation of Revenue [Line Items]
Revenues	87,917	84,399
Operating Segments | Nuclear Power Group | United States:
Disaggregation of Revenue [Line Items]
Revenues	9,608	9,162
Operating Segments | Nuclear Power Group | United States: | Government
Disaggregation of Revenue [Line Items]
Revenues		0	0
Operating Segments | Nuclear Power Group | United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues		9,162	9,608
Operating Segments | Nuclear Power Group | Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues		68,611	74,527
Operating Segments | Nuclear Power Group | Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues		6,626	3,782
Operating Segments | Nuclear Services Group
Disaggregation of Revenue [Line Items]
Revenues	36,765	29,094
Operating Segments | Nuclear Services Group | United States:
Disaggregation of Revenue [Line Items]
Revenues	35,805	28,804
Operating Segments | Nuclear Services Group | United States: | Government
Disaggregation of Revenue [Line Items]
Revenues		24,451	24,596
Operating Segments | Nuclear Services Group | United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues		4,353	11,209
Operating Segments | Nuclear Services Group | Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues		290	960
Operating Segments | Nuclear Services Group | Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues		0	$ 0
Adjustments and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	(6,249)	(1,840)
Adjustments and Eliminations | Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenues	$ (673)	$ (857)