Revenue Recognition - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 459,480
Revenues	457,463	$ 428,229
Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	253,268
Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	126,750
Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	44,231
Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	35,231
Over-time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	451,773
Point-in-time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,707
UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	345,949
UNITED STATES | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	342,512
UNITED STATES | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,437
CANADA
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	83,300
CANADA | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
CANADA | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	83,300
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,231
Other | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Other | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,231
Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	316,631
Nuclear Operations Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	249,240
Nuclear Operations Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	47,058
Nuclear Operations Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	20,233
Nuclear Operations Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	100
Nuclear Operations Group | Over-time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	316,631
Nuclear Operations Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Nuclear Services Group
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,033
Nuclear Services Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Nuclear Services Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5,410
Nuclear Services Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	23,953
Nuclear Services Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	670
Nuclear Services Group | Over-time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,033
Nuclear Services Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Nuclear Power Group
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	112,816
Nuclear Power Group | Fixed-Price Incentive Fee
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	4,028
Nuclear Power Group | Firm-Fixed-Price
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	74,282
Nuclear Power Group | Cost-Plus Fee
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	45
Nuclear Power Group | Time-and-Materials
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	34,461
Nuclear Power Group | Over-time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	105,109
Nuclear Power Group | Point-in-time
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,707
Operating Segments | Nuclear Operations Group
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	316,631
Revenues	316,631	$ 325,081
Operating Segments | Nuclear Operations Group | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	316,631
Operating Segments | Nuclear Operations Group | UNITED STATES | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	316,631
Operating Segments | Nuclear Operations Group | UNITED STATES | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Operations Group | CANADA
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Operations Group | CANADA | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Operations Group | CANADA | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Operations Group | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Operations Group | Other | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Operations Group | Other | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Services Group
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,033
Operating Segments | Nuclear Services Group | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	29,058
Operating Segments | Nuclear Services Group | UNITED STATES | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	25,881
Operating Segments | Nuclear Services Group | UNITED STATES | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3,177
Operating Segments | Nuclear Services Group | CANADA
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	975
Operating Segments | Nuclear Services Group | CANADA | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Services Group | CANADA | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	975
Operating Segments | Nuclear Services Group | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Services Group | Other | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Services Group | Other | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Power Group
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	112,816
Operating Segments | Nuclear Power Group | UNITED STATES
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	260
Operating Segments | Nuclear Power Group | UNITED STATES | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Power Group | UNITED STATES | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	260
Operating Segments | Nuclear Power Group | CANADA
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	82,325
Operating Segments | Nuclear Power Group | CANADA | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Power Group | CANADA | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	82,325
Operating Segments | Nuclear Power Group | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,231
Operating Segments | Nuclear Power Group | Other | Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0
Operating Segments | Nuclear Power Group | Other | Non-Government
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	30,231
Adjustments and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	$ (2,017)