Segment Reporting - Schedule of Operating Results by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	$ 542,208	$ 416,454
OPERATING INCOME:
Total Operating Income	98,267	63,644
Other Income (Expense)	181	(767)
Income before Provision for Income Taxes	98,448	62,877
Nuclear Operations Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	423,775	304,801
Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	548,457	418,294
OPERATING INCOME:
Total Operating Income	99,870	65,683
Operating Segments | Nuclear Operations Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	423,775	304,801
OPERATING INCOME:
Total Operating Income	90,359	57,625
Operating Segments | Nuclear Power Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	87,917	84,399
OPERATING INCOME:
Total Operating Income	8,470	12,583
Operating Segments | Nuclear Services Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	36,765	29,094
OPERATING INCOME:
Total Operating Income	6,400	1,571
Adjustments and Eliminations
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(6,249)	(1,840)
Adjustments and Eliminations | Nuclear Operations Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(673)	(857)
Adjustments and Eliminations | Nuclear Power Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(129)	(40)
Adjustments and Eliminations | Nuclear Services Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(5,447)	(943)
Segment Reconciling Items
OPERATING INCOME:
Total Operating Income	(5,359)	(6,096)
Unallocated Corporate
OPERATING INCOME:
Total Operating Income	$ (1,603)	$ (2,039)