Segment Reporting - Schedule of Operating Results by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	$ 504,520	$ 471,231	$ 1,046,728	$ 887,685
OPERATING INCOME:
Total Operating Income	82,434	80,535	180,701	144,179
Other Income (Expense)	1,646	(2,801)	1,827	(3,568)
Income before Provision for Income Taxes	84,080	77,734	182,528	140,611
Nuclear Operations Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	410,252	358,352	834,027	663,153
Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	511,563	474,820	1,060,020	893,114
OPERATING INCOME:
Total Operating Income	85,596	84,855	185,466	150,538
Operating Segments | Nuclear Operations Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	410,252	358,352	834,027	663,153
OPERATING INCOME:
Total Operating Income	85,972	75,226	176,331	132,851
Operating Segments | Nuclear Power Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	67,983	86,639	155,900	171,038
OPERATING INCOME:
Total Operating Income	1,102	14,883	9,572	27,466
Operating Segments | Nuclear Services Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	33,328	29,829	70,093	58,923
OPERATING INCOME:
Total Operating Income	4,122	1,490	10,522	3,061
Eliminations
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(7,043)	(3,589)	(13,292)	(5,429)
Eliminations | Nuclear Operations Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(863)	(1,047)	(1,536)	(1,904)
Eliminations | Nuclear Power Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(145)	(33)	(274)	(73)
Eliminations | Nuclear Services Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Revenues	(6,035)	(2,509)	(11,482)	(3,452)
Segment Reconciling Items
OPERATING INCOME:
Total Operating Income	(5,600)	(6,744)	(10,959)	(12,840)
Unallocated Corporate
OPERATING INCOME:
Total Operating Income	$ (3,162)	$ (4,320)	$ (4,765)	$ (6,359)