Revenue Recognition - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 519,878	$ 506,000	$ 1,566,606	$ 1,393,685
Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues	309,923	302,678	912,363	848,901
Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues	132,359	143,384	461,139	368,699
Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues	55,239	46,904	156,573	137,366
Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues	30,792	19,131	58,258	50,245
Over time
Disaggregation of Revenue [Line Items]
Revenues	511,125	502,226	1,553,936	1,367,630
Point-in-time
Disaggregation of Revenue [Line Items]
Revenues	17,188	9,871	34,397	37,581
United States:
Disaggregation of Revenue [Line Items]
Revenues	426,978	435,328	1,341,648	1,172,179
United States: | Government
Disaggregation of Revenue [Line Items]
Revenues	392,827	405,010	1,217,035	1,098,635
United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	34,151	30,318	124,613	73,544
Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	90,292	69,827	224,270	205,106
Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	11,043	6,942	22,415	27,926
Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenues	386,502	394,484	1,220,529	1,057,637
Nuclear Operations Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues	309,621	302,181	911,175	846,823
Nuclear Operations Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues	46,864	72,820	224,394	148,860
Nuclear Operations Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues	29,714	19,483	84,537	61,622
Nuclear Operations Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues	303	0	423	332
Nuclear Operations Group | Over time
Disaggregation of Revenue [Line Items]
Revenues	386,443	394,407	1,220,386	1,057,434
Nuclear Operations Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenues	59	77	143	203
Nuclear Power Group
Disaggregation of Revenue [Line Items]
Revenues	108,104	84,376	264,004	255,414
Nuclear Power Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues	302	497	1,188	2,078
Nuclear Power Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues	79,748	64,758	212,450	203,961
Nuclear Power Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues	0	591	0	599
Nuclear Power Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues	28,054	18,530	50,366	48,776
Nuclear Power Group | Over time
Disaggregation of Revenue [Line Items]
Revenues	90,975	74,582	229,750	218,036
Nuclear Power Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenues	17,129	9,794	34,254	37,378
Nuclear Services Group
Disaggregation of Revenue [Line Items]
Revenues	33,707	33,237	103,800	92,160
Nuclear Services Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Nuclear Services Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues	5,747	5,806	24,295	15,878
Nuclear Services Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues	25,525	26,830	72,036	75,145
Nuclear Services Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues	2,435	601	7,469	1,137
Nuclear Services Group | Over time
Disaggregation of Revenue [Line Items]
Revenues	33,707	33,237	103,800	92,160
Nuclear Services Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	528,313	512,097	1,588,333	1,405,211
Operating Segments | Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenues	386,502	394,484	1,220,529	1,057,637
Operating Segments | Nuclear Operations Group | United States:
Disaggregation of Revenue [Line Items]
Revenues	384,726	392,650	1,214,777	1,052,951
Operating Segments | Nuclear Operations Group | United States: | Government
Disaggregation of Revenue [Line Items]
Revenues	362,149	376,243	1,133,275	1,016,594
Operating Segments | Nuclear Operations Group | United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	22,577	16,407	81,502	36,357
Operating Segments | Nuclear Operations Group | Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Nuclear Operations Group | Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	1,776	1,834	5,752	4,686
Operating Segments | Nuclear Power Group
Disaggregation of Revenue [Line Items]
Revenues	108,104	84,376	264,004	255,414
Operating Segments | Nuclear Power Group | United States:
Disaggregation of Revenue [Line Items]
Revenues	9,479	10,130	26,076	28,265
Operating Segments | Nuclear Power Group | United States: | Government
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Nuclear Power Group | United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	9,479	10,130	26,076	28,265
Operating Segments | Nuclear Power Group | Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	89,358	69,138	221,265	203,909
Operating Segments | Nuclear Power Group | Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	9,267	5,108	16,663	23,240
Operating Segments | Nuclear Services Group
Disaggregation of Revenue [Line Items]
Revenues	33,707	33,237	103,800	92,160
Operating Segments | Nuclear Services Group | United States:
Disaggregation of Revenue [Line Items]
Revenues	32,773	32,548	100,795	90,963
Operating Segments | Nuclear Services Group | United States: | Government
Disaggregation of Revenue [Line Items]
Revenues	30,678	28,767	83,760	82,041
Operating Segments | Nuclear Services Group | United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	2,095	3,781	17,035	8,922
Operating Segments | Nuclear Services Group | Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	934	689	3,005	1,197
Operating Segments | Nuclear Services Group | Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Eliminations
Disaggregation of Revenue [Line Items]
Revenues	(8,435)	(6,097)	(21,727)	(11,526)
Eliminations | Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenues	$ (970)	$ (1,260)	$ (2,506)	$ (3,164)