Revenue - Disaggregation of revenue by customer type and contract type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Disaggregation of Revenue
Revenue	$ 1,952.4	$ 2,178.4	$ 5,200.5	$ 5,717.3
Fixed Price
Disaggregation of Revenue
Revenue	657.7	1,036.9	1,992.9	2,850.6
Unit Price
Disaggregation of Revenue
Revenue	674.5	619.4	1,891.6	1,691.7
Cost Reimbursable
Disaggregation of Revenue
Revenue	620.2	522.1	1,316.0	1,175.0
MSA
Disaggregation of Revenue
Revenue	835.0	675.7	2,115.4	1,768.7
Non-MSA
Disaggregation of Revenue
Revenue	1,117.4	1,502.7	3,085.1	3,948.6
Intersegment eliminations
Disaggregation of Revenue
Revenue	(42.3)	(44.8)	(95.0)	(107.5)
Intersegment eliminations \| Fixed Price
Disaggregation of Revenue
Revenue	(34.7)	(32.6)	(77.3)	(83.8)
Intersegment eliminations \| Unit Price
Disaggregation of Revenue
Revenue		(5.6)	(0.7)	(15.5)
Intersegment eliminations \| Cost Reimbursable
Disaggregation of Revenue
Revenue	(7.6)	(6.6)	(17.0)	(8.2)
Intersegment eliminations \| MSA
Disaggregation of Revenue
Revenue	(18.4)	(4.8)	(18.4)	(5.7)
Intersegment eliminations \| Non-MSA
Disaggregation of Revenue
Revenue	(23.9)	(40.0)	(76.6)	(101.8)
Utilities \| Operating Segment
Disaggregation of Revenue
Revenue	770.9	737.5	2,116.4	1,993.9
Utilities \| Operating Segment \| Fixed Price
Disaggregation of Revenue
Revenue	116.6	118.3	335.6	306.8
Utilities \| Operating Segment \| Unit Price
Disaggregation of Revenue
Revenue	507.6	466.8	1,382.4	1,254.9
Utilities \| Operating Segment \| Cost Reimbursable
Disaggregation of Revenue
Revenue	146.7	152.4	398.4	432.2
Utilities \| Operating Segment \| MSA
Disaggregation of Revenue
Revenue	721.9	555.1	1,779.0	1,546.6
Utilities \| Operating Segment \| Non-MSA
Disaggregation of Revenue
Revenue	49.0	182.4	337.4	447.3
Utilities \| Intersegment eliminations
Disaggregation of Revenue
Revenue	24.0	44.6	76.6	107.3
Energy \| Operating Segment
Disaggregation of Revenue
Revenue	1,223.8	1,485.7	3,179.1	3,830.9
Energy \| Operating Segment \| Fixed Price
Disaggregation of Revenue
Revenue	575.8	951.2	1,734.6	2,627.6
Energy \| Operating Segment \| Unit Price
Disaggregation of Revenue
Revenue	166.9	158.2	509.9	452.3
Energy \| Operating Segment \| Cost Reimbursable
Disaggregation of Revenue
Revenue	481.1	376.3	934.6	751.0
Energy \| Operating Segment \| MSA
Disaggregation of Revenue
Revenue	131.5	125.4	354.8	227.8
Energy \| Operating Segment \| Non-MSA
Disaggregation of Revenue
Revenue	1,092.3	1,360.3	2,824.3	3,603.1
Energy \| Intersegment eliminations
Disaggregation of Revenue
Revenue	$ 18.3	$ 0.2	$ 18.4	$ 0.2