Segment Reporting - Schedule of Operating Results by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 686,006	$ 886,136	$ 1,348,023	$ 1,691,559
Special Charges for Restructuring Activities	(17,470)	(12,232)	(20,128)	(20,655)
Operating income	35,118	98,706	88,758	158,919
Interest income	190	323	609	655
Losses (gains) on asset disposals and impairments	1,457	156	1,457	87
Interest expense	(921)	(789)	(1,820)	(1,607)
Equity in Income of Investees	13,183	18,775	28,452	33,562
Other - net	580	1,005	1,902	2,411
Total Other Income (Expense)	(151)	539	691	1,459
Income before Provision for Income Taxes	34,967	99,245	89,449	160,378
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Special Charges for Restructuring Activities	(17,470)	(12,232)	(20,128)	(20,655)
Operating income	35,118	98,706	88,758	158,919
Interest income	190	323	609	655
Interest expense	(921)	(789)	(1,820)	(1,607)
Other - net	580	1,005	1,902	2,411
Total Other Income (Expense)	(151)	539	691	1,459
Operating Segments [Member] | Power Generation [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	327,379	471,191	639,457	932,654
Operating income	15,215	30,535	25,757	63,865
Losses (gains) on asset disposals and impairments	1,457	(7)	1,457	(85)
Equity in Income of Investees	434	5,202	2,800	7,309
Operating Segments [Member] | Nuclear Operations [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	293,438	330,986	579,652	592,125
Operating income	58,682	65,737	118,210	120,461
Losses (gains) on asset disposals and impairments
Equity in Income of Investees
Operating Segments [Member] | Technical Services [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	26,015	27,432	50,470	52,661
Operating income	15,078	15,235	29,867	29,414
Losses (gains) on asset disposals and impairments		163		163
Equity in Income of Investees	12,749	13,699	25,650	26,532
Operating Segments [Member] | Nuclear Energy [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	44,927	63,185	92,707	126,701
Operating income	1,548	7,922	2,071	10,180
Losses (gains) on asset disposals and impairments				9
Equity in Income of Investees		(126)	2	(279)
Operating Segments [Member] | mPower [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues		333	278	637
Operating income	(31,933)	(1,104)	(58,642)	(28,051)
Losses (gains) on asset disposals and impairments
Equity in Income of Investees
Operating Segments [Member] | Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating income	58,590	118,325	117,263	195,869
Adjustments and Eliminations [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(5,753)	(6,991)	(14,541)	(13,219)
Adjustments and Eliminations [Member] | Power Generation [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(1,239)	(1,538)	(4,220)	(2,300)
Adjustments and Eliminations [Member] | Nuclear Operations [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(1,880)	(1,539)	(4,967)	(2,816)
Adjustments and Eliminations [Member] | Technical Services [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues		(1,014)	(52)	(1,549)
Adjustments and Eliminations [Member] | Nuclear Energy [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(2,634)	(2,900)	(5,302)	(6,554)
Adjustments and Eliminations [Member] | mPower [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues
Unallocated Corporate [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Operating income	$ (6,002)	$ (7,387)	$ (8,377)	$ (16,295)