Revenue Recognition - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 504,520	$ 471,231	$ 1,046,728	$ 887,685
Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues	317,574	300,463	602,440	546,223
Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues	132,970	117,806	328,780	225,315
Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues	52,845	48,074	101,334	90,462
Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues	8,174	8,477	27,466	31,114
Over time
Disaggregation of Revenue [Line Items]
Revenues	505,122	460,916	1,042,811	865,404
Point-in-time
Disaggregation of Revenue [Line Items]
Revenues	6,441	13,904	17,209	27,710
United States:
Disaggregation of Revenue [Line Items]
Revenues	447,350	396,244	914,670	736,851
United States: | Government
Disaggregation of Revenue [Line Items]
Revenues	398,546	371,871	824,208	693,625
United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	48,804	24,373	90,462	43,226
Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	58,491	66,378	133,978	135,279
Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	5,722	12,198	11,372	20,984
Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenues	410,252	358,352	834,027	663,153
Nuclear Operations Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues	317,180	299,155	601,554	544,642
Nuclear Operations Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues	64,193	36,697	177,530	76,040
Nuclear Operations Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues	28,827	22,286	54,823	42,139
Nuclear Operations Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues	52	214	120	332
Nuclear Operations Group | Over time
Disaggregation of Revenue [Line Items]
Revenues	410,204	358,294	833,943	663,027
Nuclear Operations Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenues	48	58	84	126
Nuclear Power Group
Disaggregation of Revenue [Line Items]
Revenues	67,983	86,639	155,900	171,038
Nuclear Power Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenues	394	1,308	886	1,581
Nuclear Power Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenues	61,866	77,205	132,702	139,203
Nuclear Power Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues	0	8	0	8
Nuclear Power Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues	5,723	8,118	22,312	30,246
Nuclear Power Group | Over time
Disaggregation of Revenue [Line Items]
Revenues	61,590	72,793	138,775	143,454
Nuclear Power Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenues	6,393	13,846	17,125	27,584
Nuclear Services Group
Disaggregation of Revenue [Line Items]
Revenues	33,328	29,829	70,093	58,923
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	6,911	3,904	18,548	10,072
Nuclear Services Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenues	24,018	25,780	46,511	48,315
Nuclear Services Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenues	2,399	145	5,034	536
Nuclear Services Group | Over time
Disaggregation of Revenue [Line Items]
Revenues	33,328	29,829	70,093	58,923
Nuclear Services Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	511,563	474,820	1,060,020	893,114
Operating Segments | Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenues	410,252	358,352	834,027	663,153
Operating Segments | Nuclear Operations Group | United States:
Disaggregation of Revenue [Line Items]
Revenues	408,144	357,660	830,051	660,301
Operating Segments | Nuclear Operations Group | United States: | Government
Disaggregation of Revenue [Line Items]
Revenues	370,060	343,048	771,126	640,351
Operating Segments | Nuclear Operations Group | United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	38,084	14,612	58,925	19,950
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	2,108	692	3,976	2,852
Operating Segments | Nuclear Power Group
Disaggregation of Revenue [Line Items]
Revenues	67,983	86,639	155,900	171,038
Operating Segments | Nuclear Power Group | United States:
Disaggregation of Revenue [Line Items]
Revenues	6,989	8,973	16,597	18,135
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	6,989	8,973	16,597	18,135
Operating Segments | Nuclear Power Group | Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	57,380	66,160	131,907	134,771
Operating Segments | Nuclear Power Group | Other: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	3,614	11,506	7,396	18,132
Operating Segments | Nuclear Services Group
Disaggregation of Revenue [Line Items]
Revenues	33,328	29,829	70,093	58,923
Operating Segments | Nuclear Services Group | United States:
Disaggregation of Revenue [Line Items]
Revenues	32,217	29,611	68,022	58,415
Operating Segments | Nuclear Services Group | United States: | Government
Disaggregation of Revenue [Line Items]
Revenues	28,486	28,823	53,082	53,274
Operating Segments | Nuclear Services Group | United States: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	3,731	788	14,940	5,141
Operating Segments | Nuclear Services Group | Canada: | Non-Government
Disaggregation of Revenue [Line Items]
Revenues	1,111	218	2,071	508
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	(7,043)	(3,589)	(13,292)	(5,429)
Eliminations | Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenues	$ (863)	$ (1,047)	$ (1,536)	$ (1,904)