Segment Reporting - Schedule of Operating Results by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	$ 457,463	$ 428,229
Operating Income	79,888	76,272
Interest income	778	137
Interest expense	(3,560)	(3,517)
Other – net	7,910	7,486
Total Other Income (Expense)	5,128	4,106
Income before Provision for Income Taxes	85,016	80,378
Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating Income	86,555	79,495
Operating Segments | Nuclear Operations Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	316,631	325,081
Operating Income	67,657	67,749
Operating Segments | Nuclear Services Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	30,033	27,854
Operating Income	1,177	402
Operating Segments | Nuclear Power Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	112,816	77,674
Operating Income	21,764	12,956
Operating Segments | Other
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating Income	(4,043)	(1,612)
Adjustments and Eliminations
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(2,017)	(2,380)
Adjustments and Eliminations | Nuclear Operations Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(1,139)	(195)
Adjustments and Eliminations | Nuclear Services Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(838)	(2,157)
Adjustments and Eliminations | Nuclear Power Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(40)	(28)
Unallocated Corporate
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating Income	$ (6,667)	$ (3,223)