Operating Information by Segment and Geographic Area - Narrative (Details) $ / shares in Units, $ in Thousands, $ in Millions		3 Months Ended			6 Months Ended		9 Months Ended
	Dec. 13, 2016 AUD ($)	Jun. 30, 2020 USD ($) $ / shares	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($) $ / shares	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($) project $ / shares	Sep. 30, 2019 USD ($) $ / shares	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Assets		$ 7,240,774			$ 7,240,774			$ 7,966,658
Revenue		4,090,993		$ 4,146,404	8,209,549	$ 8,280,016
Energy Chemicals And Mining Segment
Segment Reporting Information [Line Items]
Impairment of equity method investments			$ 86,000		86,000
Energy Chemicals And Mining Segment | Credit Losses Associated with Clients Affected by Impacts of COVID-19
Segment Reporting Information [Line Items]
Reserve on receivables		55,000			55,000
Energy & Chemicals
Segment Reporting Information [Line Items]
Change in cost and schedule impacts				$ 87,000	$ 87,000	$ 31,000
Change in cost and schedule impacts (in dollars per share) | $ / shares				$ 0.50	$ 0.50	$ 0.22
Energy & Chemicals | Write-off of pre-contract costs
Segment Reporting Information [Line Items]
Charges from write-off of pre-contract costs				$ 26,000			$ 26,000
Charges from write-off of pre-contract costs, per diluted share (in dollars per share) | $ / shares				$ 0.15
Change in cost and schedule impacts (in dollars per share) | $ / shares							$ 0.15
Diversified Services
Segment Reporting Information [Line Items]
Impairment of goodwill			169,000		$ 169,000
Impairment of intangible customer relationships			$ 27,000		27,000
Decrease in accounts receivable and contract assets					101,000
Other
Segment Reporting Information [Line Items]
Assets		37,200			37,200			68,500
Other | Warren project
Segment Reporting Information [Line Items]
Effect of forecast revision on estimated project cost				$ 57,000
Effect of forecast revision on estimated project cost (in dollars per share) | $ / shares				$ 0.32
Pending litigation
Segment Reporting Information [Line Items]
Maximum exposure	$ 1,470
United States | Energy Chemicals And Mining Segment
Segment Reporting Information [Line Items]
Number of downstream projects | project						2
United States | Energy Chemicals And Mining Segment | Forecast revisions resulting from late design changes, schedule-driven cost growth, and subcontractor negotiations
Segment Reporting Information [Line Items]
Pre-tax charges adversely affecting earnings before taxes				$ 179,000		$ 240,000
Pre-tax charges adversely affecting earnings before taxes (in dollars per share) | $ / shares				$ 1.07		$ 1.40
Corporate general and administrative expense
Segment Reporting Information [Line Items]
Write-off of cumulative translation loss					45,000	$ 18,000
Reportable segments
Segment Reporting Information [Line Items]
Revenue		4,091,000		$ 4,146,400	8,209,500	8,280,000
Reportable segments | Energy & Chemicals
Segment Reporting Information [Line Items]
Assets		961,500			961,500			1,139,300
Revenue		1,495,900		1,398,900	2,851,400	$ 2,873,600
Reportable segments | Infrastructure & Power
Segment Reporting Information [Line Items]
Number of downstream projects | project						3
Revenue		474,700		229,400	860,500	$ 573,400
Reportable segments | Infrastructure & Power | Settlement of client disputes as well as cost growth related to close-out matters
Segment Reporting Information [Line Items]
Effect of forecast revision on estimated project cost				$ 109,000		$ 135,000
Effect of forecast revision on estimated project cost (in dollars per share) | $ / shares				$ 0.60		$ 0.74
Reportable segments | Infrastructure & Power | Late engineering changes
Segment Reporting Information [Line Items]
Effect of forecast revision on estimated project cost		$ 55,000				$ 55,000
Effect of forecast revision on estimated project cost (in dollars per share) | $ / shares		$ 0.30				$ 0.30
Reportable segments | Diversified Services
Segment Reporting Information [Line Items]
Assets		$ 951,800			951,800			$ 1,290,600
Revenue		382,900		$ 514,900	840,100	$ 1,003,300
Reportable segments | Other
Segment Reporting Information [Line Items]
Revenue		22,700		3,000	51,200	22,100
Reportable segments | NuScale | Other
Segment Reporting Information [Line Items]
Research and development activities		18,000		18,000	41,000	34,000
Intersegment Eliminations | Diversified Services
Segment Reporting Information [Line Items]
Revenue		62,000		92,000	140,000	172,000
Research And Development Activities Cost Sharing Agreement | Reportable segments | DOE | Total cost of revenue | NuScale | Other
Segment Reporting Information [Line Items]
Net of qualified reimbursable expenses		$ 20,000		$ 15,000	$ 33,000	$ 28,000