Revenue - Disaggregation of revenue by customer type and contract type (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue
Revenue	$ 1,688.2	$ 1,890.7	$ 3,248.1	$ 3,538.8
Fixed Price
Disaggregation of Revenue
Revenue	634.0	977.8	1,335.4	1,813.5
Unit Price
Disaggregation of Revenue
Revenue	659.6	584.3	1,216.9	1,072.4
Cost Reimbursable
Disaggregation of Revenue
Revenue	394.6	328.6	695.8	652.9
MSA
Disaggregation of Revenue
Revenue	720.6	602.9	1,280.4	1,092.9
Non-MSA
Disaggregation of Revenue
Revenue	967.6	1,287.8	1,967.7	2,445.9
Intersegment eliminations
Disaggregation of Revenue
Revenue	(24.3)	(39.1)	(52.7)	(62.7)
Intersegment eliminations \| Fixed Price
Disaggregation of Revenue
Revenue	(16.7)	(32.1)	(42.4)	(51.3)
Intersegment eliminations \| Unit Price
Disaggregation of Revenue
Revenue	0.7	(6.5)	(0.9)	(9.9)
Intersegment eliminations \| Cost Reimbursable
Disaggregation of Revenue
Revenue	(8.3)	(0.5)	(9.4)	(1.5)
Intersegment eliminations \| MSA
Disaggregation of Revenue
Revenue		(0.4)		(0.9)
Intersegment eliminations \| Non-MSA
Disaggregation of Revenue
Revenue	(24.3)	(38.7)	(52.7)	(61.8)
Utilities \| Operating Segment
Disaggregation of Revenue
Revenue	712.6	693.0	1,345.5	1,256.4
Utilities \| Operating Segment \| Fixed Price
Disaggregation of Revenue
Revenue	106.9	110.5	219.0	188.5
Utilities \| Operating Segment \| Unit Price
Disaggregation of Revenue
Revenue	475.7	443.8	874.8	788.1
Utilities \| Operating Segment \| Cost Reimbursable
Disaggregation of Revenue
Revenue	130.0	138.7	251.7	279.8
Utilities \| Operating Segment \| MSA
Disaggregation of Revenue
Revenue	583.5	539.4	1,057.1	991.5
Utilities \| Operating Segment \| Non-MSA
Disaggregation of Revenue
Revenue	129.1	153.6	288.4	264.9
Utilities \| Intersegment eliminations
Disaggregation of Revenue
Revenue	24.2	39.1	52.6	62.7
Energy \| Operating Segment
Disaggregation of Revenue
Revenue	999.9	1,236.8	1,955.3	2,345.1
Energy \| Operating Segment \| Fixed Price
Disaggregation of Revenue
Revenue	543.8	899.4	1,158.8	1,676.3
Energy \| Operating Segment \| Unit Price
Disaggregation of Revenue
Revenue	183.2	147.0	343.0	294.2
Energy \| Operating Segment \| Cost Reimbursable
Disaggregation of Revenue
Revenue	272.9	190.4	453.5	374.6
Energy \| Operating Segment \| MSA
Disaggregation of Revenue
Revenue	137.1	63.9	223.3	102.3
Energy \| Operating Segment \| Non-MSA
Disaggregation of Revenue
Revenue	862.8	$ 1,172.9	1,732.0	$ 2,242.8
Energy \| Intersegment eliminations
Disaggregation of Revenue
Revenue	$ 0.1		$ 0.1